[dev]
dev ansible_ssh_host=ec2-host-ip.compute-1.amazonaws.com ansible_ssh_user=ec2-user ansible_ssh_private_key_file=~/awskeys/ec2key.pem

[vagrant]
web ansible_ssh_host=192.168.33.20 ansible_ssh_user=vagrant ansible_ssh_pass=vagrant
db ansible_ssh_host=192.168.33.30 ansible_ssh_user=vagrant ansible_ssh_pass=vagrant

$ ansible vagrant -i invertory -m ping

$ ansible dev-wb -i invertory -m ping -vvv

Using /etc/ansible/ansible.cfg as config file
<ec2-some-ip.compute-1.amazonaws.com> ESTABLISH SSH CONNECTION FOR USER: ec2-user
<ec2-some-ip.compute-1.amazonaws.com> SSH: EXEC ssh -C -q -o ControlMaster=auto -o ControlPersist=60s -o 'IdentityFile="~/awskeys/ec2key.pem"' -o KbdInteractiveAuthentication=no -o PreferredAuthentications=gssapi-with-mic,gssapi-keyex,hostbased,publickey -o PasswordAuthentication=no -o User=ec2-user -o ConnectTimeout=10 -o ControlPath=/Users/.ansible/cp/ansible-ssh-%h-%p-%r ec2-some-ip.compute-1.amazonaws.com '/bin/sh -c '"'"'( umask 77 && mkdir -p "` echo $HOME/.ansible/tmp/ansible-tmp-1469435660.51-156184209634917 `" && echo ansible-tmp-1469435660.51-156184209634917="` echo $HOME/.ansible/tmp/ansible-tmp-1469435660.51-156184209634917 `" ) && sleep 0'"'"''
dev-wb | UNREACHABLE! => {
    "changed": false,
    "msg": "Failed to connect to the host via ssh.",
    "unreachable": true
}

$ ssh -t -i "~/awskeys/ec2key.pem" ec2-user@ec2-some-ip.compute-1.amazonaws.com -p 22