Group Variables
You can set variables that apply to all hosts by using the playbook layout specified in Ansible's Best Practices document and creating a group_vars/all file where you define them.

---
# file: group_vars/all
ansible_connection: ssh
ansible_ssh_user: root
ansible_ssh_pass: root

abhishek@bjlinux028 ~/Downloads/android-studio/bin> ansible all -m ping -vvvv
/usr/lib64/python2.6/site-packages/pycrypto-2.6.1-py2.6-linux-x86_64.egg/Crypto/Util/number.py:57: PowmInsecureWarning: Not using mpz_powm_sec.  You should rebuild using libgmp >= 5 to avoid timing attack vulnerability.
  _warn("Not using mpz_powm_sec.  You should rebuild using libgmp >= 5 to avoid timing attack vulnerability.", PowmInsecureWarning)
Using /etc/ansible/ansible.cfg as config file
Loaded callback minimal of type stdout, v2.0
<172.16.15.246> ESTABLISH SSH CONNECTION FOR USER: root
<172.16.15.246> SSH: EXEC ssh -C -vvv -o ControlMaster=auto -o ControlPersist=60s -o KbdInteractiveAuthentication=no -o PreferredAuthentications=gssapi-with-mic,gssapi-keyex,hostbased,publickey -o PasswordAuthentication=no -o User=root -o ConnectTimeout=10 -o ControlPath=/home/abhishek/.ansible/cp/ansible-ssh-%h-%p-%r -tt 172.16.15.246 '/bin/sh -c '"'"'mkdir -p "` echo $HOME/.ansible/tmp/ansible-tmp-1457459683.9-19018413779551 `" && echo "` echo $HOME/.ansible/tmp/ansible-tmp-1457459683.9-19018413779551 `"'"'"''
172.16.15.246 | UNREACHABLE! => {
    "changed": false,
    "msg": "SSH encountered an unknown error. The output was:\nOpenSSH_6.7p1 (CentrifyDC build 5.2.3-425) , OpenSSL 0.9.8zf-fips 19 Mar 2015\r\ndebug1: Reading configuration data /home/abhishek/.ssh/config\r\ndebug1: /home/abhishek/.ssh/config line 1: Applying options for *\r\ndebug1: Reading configuration data /etc/centrifydc/ssh/ssh_config\r\ndebug1: /etc/centrifydc/ssh/ssh_config line 49: Applying options for *\r\ndebug1: auto-mux: Trying existing master\r\ndebug1: Control socket \"/home/abhishek/.ansible/cp/ansible-ssh-172.16.15.246-22-root\" does not exist\r\ndebug2: ssh_connect: needpriv 0\r\ndebug1: Connecting to 172.16.15.246 [172.16.15.246] port 22.\r\ndebug2: fd 3 setting O_NONBLOCK\r\ndebug1: fd 3 clearing O_NONBLOCK\r\ndebug1: Connection established.\r\ndebug3: timeout: 10000 ms remain after connect\r\ndebug1: identity file /home/abhishek/.ssh/id_rsa type 1\r\ndebug1: key_load_public: No such file or directory\r\ndebug1: identity file /home/abhishek/.ssh/id_rsa-cert type -1\r\ndebug1: key_load_public: No such file or directory\r\ndebug1: identity file /home/abhishek/.ssh/id_dsa type -1\r\ndebug1: key_load_public: No such file or directory\r\ndebug1: identity file /home/abhishek/.ssh/id_dsa-cert type -1\r\ndebug1: key_load_public: No such file or directory\r\ndebug1: identity file /home/abhishek/.ssh/id_ecdsa type -1\r\ndebug1: key_load_public: No such file or directory\r\ndebug1: identity file /home/abhishek/.ssh/id_ecdsa-cert type -1\r\ndebug1: key_load_public: No such file or directory\r\ndebug1: identity file /home/abhishek/.ssh/id_ed25519 type -1\r\ndebug1: key_load_public: No such file or directory\r\ndebug1: identity file /home/abhishek/.ssh/id_ed25519-cert type -1\r\ndebug1: Enabling compatibility mode for protocol 2.0\r\ndebug1: Local version string SSH-2.0-OpenSSH_6.7\r\ndebug1: Remote protocol version 2.0, remote software version OpenSSH_6.7\r\ndebug1: match: OpenSSH_6.7 pat OpenSSH* compat 0x04000000\r\ndebug2: fd 3 setting O_NONBLOCK\r\ndebug1: Miscellaneous failure\nMatching credential not found\n\r\ndebug1: Miscellaneous failure\nMatching credential not found\n\r\ndebug3: load_hostkeys: loading entries for host \"172.16.15.246\" from file \"/home/abhishek/.ssh/known_hosts\"\r\ndebug3: load_hostkeys: found key type RSA in file /home/abhishek/.ssh/known_hosts:81\r\ndebug3: load_hostkeys: loaded 1 keys\r\ndebug3: order_hostkeyalgs: prefer hostkeyalgs: ssh-rsa-cert-v01@openssh.com,ssh-rsa-cert-v00@openssh.com,ssh-rsa\r\ndebug1: SSH2_MSG_KEXINIT sent\r\ndebug1: SSH2_MSG_KEXINIT received\r\ndebug2: kex_parse_kexinit: curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha1\r\ndebug2: kex_parse_kexinit: ssh-rsa-cert-v01@openssh.com,ssh-rsa-cert-v00@openssh.com,ssh-rsa,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,ssh-dss-cert-v00@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519,ssh-dss\r\ndebug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,chacha20-poly1305@openssh.com,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se\r\ndebug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,chacha20-poly1305@openssh.com,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se\r\ndebug2: kex_parse_kexinit: umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-md5-etm@openssh.com,hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,hmac-md5,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96\r\ndebug2: kex_parse_kexinit: umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-md5-etm@openssh.com,hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,hmac-md5,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96\r\ndebug2: kex_parse_kexinit: zlib@openssh.com,zlib,none\r\ndebug2: kex_parse_kexinit: zlib@openssh.com,zlib,none\r\ndebug2: kex_parse_kexinit: \r\ndebug2: kex_parse_kexinit: \r\ndebug2: kex_parse_kexinit: first_kex_follows 0 \r\ndebug2: kex_parse_kexinit: reserved 0 \r\ndebug2: kex_parse_kexinit: gss-gex-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group1-sha1-toWM5Slw5Ew8Mqkay+al2g==,gss-group14-sha1-toWM5Slw5Ew8Mqkay+al2g==,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1\r\ndebug2: kex_parse_kexinit: ssh-rsa,ssh-dss,ecdsa-sha2-nistp256,ssh-ed25519\r\ndebug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,chacha20-poly1305@openssh.com\r\ndebug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,chacha20-poly1305@openssh.com\r\ndebug2: kex_parse_kexinit: umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1\r\ndebug2: kex_parse_kexinit: umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1\r\ndebug2: kex_parse_kexinit: none,zlib@openssh.com\r\ndebug2: kex_parse_kexinit: none,zlib@openssh.com\r\ndebug2: kex_parse_kexinit: \r\ndebug2: kex_parse_kexinit: \r\ndebug2: kex_parse_kexinit: first_kex_follows 0 \r\ndebug2: kex_parse_kexinit: reserved 0 \r\ndebug2: mac_setup: setup umac-64-etm@openssh.com\r\ndebug1: kex: server->client aes128-ctr umac-64-etm@openssh.com zlib@openssh.com\r\ndebug2: mac_setup: setup umac-64-etm@openssh.com\r\ndebug1: kex: client->server aes128-ctr umac-64-etm@openssh.com zlib@openssh.com\r\ndebug1: sending SSH2_MSG_KEX_ECDH_INIT\r\ndebug1: expecting SSH2_MSG_KEX_ECDH_REPLY\r\ndebug1: Server host key: RSA 7b:8a:a4:2a:7b:9b:85:89:8a:c1:7c:48:5e:d9:c5:a5\r\ndebug3: load_hostkeys: loading entries for host \"172.16.15.246\" from file \"/home/abhishek/.ssh/known_hosts\"\r\ndebug3: load_hostkeys: found key type RSA in file /home/abhishek/.ssh/known_hosts:81\r\ndebug3: load_hostkeys: loaded 1 keys\r\ndebug1: Host '172.16.15.246' is known and matches the RSA host key.\r\ndebug1: Found key in /home/abhishek/.ssh/known_hosts:81\r\ndebug2: kex_derive_keys\r\ndebug2: set_newkeys: mode 1\r\ndebug1: SSH2_MSG_NEWKEYS sent\r\ndebug1: expecting SSH2_MSG_NEWKEYS\r\ndebug2: set_newkeys: mode 0\r\ndebug1: SSH2_MSG_NEWKEYS received\r\ndebug1: Roaming not allowed by server\r\ndebug1: SSH2_MSG_SERVICE_REQUEST sent\r\ndebug2: service_accept: ssh-userauth\r\ndebug1: SSH2_MSG_SERVICE_ACCEPT received\r\ndebug2: key: /home/abhishek/.ssh/id_rsa (0x11a95e0),\r\ndebug2: key: /home/abhishek/.ssh/id_dsa ((nil)),\r\ndebug2: key: /home/abhishek/.ssh/id_ecdsa ((nil)),\r\ndebug2: key: /home/abhishek/.ssh/id_ed25519 ((nil)),\r\ndebug3: input_userauth_banner\r\nCentOS release 6.4 (Final)\nKernel 2.6.32-431.11.2.el6.x86_64 on an x86_64\n\ndebug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password,keyboard-interactive\r\ndebug3: start over, passed a different list publickey,gssapi-keyex,gssapi-with-mic,password,keyboard-interactive\r\ndebug3: preferred gssapi-with-mic,gssapi-keyex,hostbased,publickey\r\ndebug3: authmethod_lookup gssapi-with-mic\r\ndebug3: remaining preferred: gssapi-keyex,hostbased,publickey\r\ndebug3: authmethod_is_enabled gssapi-with-mic\r\ndebug1: Next authentication method: gssapi-with-mic\r\ndebug1: Miscellaneous failure\nMatching credential not found\n\r\ndebug1: Miscellaneous failure\nMatching credential not found\n\r\ndebug2: we did not send a packet, disable method\r\ndebug3: authmethod_lookup gssapi-keyex\r\ndebug3: remaining preferred: hostbased,publickey\r\ndebug3: authmethod_is_enabled gssapi-keyex\r\ndebug1: Next authentication method: gssapi-keyex\r\ndebug1: No valid Key exchange context\r\ndebug2: we did not send a packet, disable method\r\ndebug3: authmethod_lookup publickey\r\ndebug3: remaining preferred: ,publickey\r\ndebug3: authmethod_is_enabled publickey\r\ndebug1: Next authentication method: publickey\r\ndebug1: Offering RSA public key: /home/abhishek/.ssh/id_rsa\r\ndebug3: send_pubkey_test\r\ndebug2: we sent a publickey packet, wait for reply\r\ndebug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password,keyboard-interactive\r\ndebug1: Trying private key: /home/abhishek/.ssh/id_dsa\r\ndebug3: no such identity: /home/abhishek/.ssh/id_dsa: No such file or directory\r\ndebug1: Trying private key: /home/abhishek/.ssh/id_ecdsa\r\ndebug3: no such identity: /home/abhishek/.ssh/id_ecdsa: No such file or directory\r\ndebug1: Trying private key: /home/abhishek/.ssh/id_ed25519\r\ndebug3: no such identity: /home/abhishek/.ssh/id_ed25519: No such file or directory\r\ndebug2: we did not send a packet, disable method\r\ndebug1: No more authentication methods to try.\r\nPermission denied (publickey,gssapi-keyex,gssapi-with-mic,password,keyboard-interactive).\r\n",
    "unreachable": true
}
abhishek@bjlinux028 ~/Downloads/android-studio/bin> cat /etc/ansible/group_vars/all
---
ansible_ssh_user: root