- name: set up mongodb credentials
  mongodb_user: name=new_db_user password=user_pwd database=mydb roles='readWrite' state=present login_user=admin login_password=admin_pwd login_database=admin

fatal: [some.host.net]: FAILED! => {
"changed": false,
  "failed": true,
  "invocation": {
    "module_args": {
      "database": "mydb",
      "login_database": "admin",
      "login_host": "localhost",
      "login_password": "admin_pwd",
      "login_port": "27017",
      "login_user": "admin",
      "name": "new_db_user",
      "password": "user_pwd",
      "replica_set": null,
      "roles": [
        "readWrite"
      ],
      "ssl": false,
      "state": "present",
      "update_password": "always"
    },
    "module_name": "mongodb_user"
  },
  "msg": "not authorized for insert on mydb.system.users"
}

[root@somehost etc]# mongo admin -u admin -p
MongoDB shell version: 2.6.9
Enter password:
connecting to: admin
> use mydb
switched to db mydb
> db.createUser({user: 'new_db_user', pwd: 'user_pwd', roles: ['readWrite']})
Successfully added user: { "user" : "new_db_user", "roles" : [ "readWrite" ] }

ansible -i hosts mongo --user vagrant -k -m  name=new_db_user -m mongodb_user -a "name=new_db_user password=user_pwd database=mydb roles='readWrite' state=present login_user=admin1 login_password=admin_pwd1 login_database=admin"
SSH password:
local | SUCCESS => {
    "changed": true,
    "user": "new_db_user"
}