Description of problem:
Follow the document /usr/share/ansible/openshift-ansible/playbooks/aws/README.md to provisioning cluster in AWS with none-root user(ec2-user). Set "ansible_ssh_user=ec2-user" in hosts file, it has no effect because ansible_ssh_user overwritten by openshift_aws_build_ami_ssh_user in /usr/share/ansible/openshift-ansible/playbooks/aws/openshift-cluster/build_ami.yml:37.

Version-Release number of the following components:
openshift-ansible-3.7.0-0.148.0.git.0.b35eb14.el7
ansible-2.3.2.0-2.el7

How reproducible:
100%

Steps to Reproduce:
1. Follow the document /usr/share/ansible/openshift-ansible/playbooks/aws/README.md to provisioning cluster in AWS with none-root user(ec2-user).
# cat hosts
...
[OSEv3:vars]
ansible_ssh_user=ec2-user
...
2. Run provisioning playbook
# ansible-playbook -i hosts -vvv /usr/share/ansible/openshift-ansible/playbooks/aws/openshift-cluster/build_ami.yml -e @provisioning_vars.yml
3.

Actual results:
# ansible-playbook -i hosts -vvv /usr/share/ansible/openshift-ansible/playbooks/aws/openshift-cluster/build_ami.yml -e @provisioning_vars.yml
...
TASK [set the user to perform installation] **************************************************************************************************************************************************
task path: /usr/share/ansible/openshift-ansible/playbooks/aws/openshift-cluster/build_ami.yml:35
ok: [xxx.compute-1.amazonaws.com] => {
    "ansible_facts": {
        "ansible_ssh_user": "root"
    }, 
    "changed": false
}
META: ran handlers
META: ran handlers
...
TASK [Gathering Facts] ***********************************************************************************************************************************************************************
Using module file /usr/lib/python2.7/site-packages/ansible/modules/system/setup.py
<xxx.compute-1.amazonaws.com> ESTABLISH SSH CONNECTION FOR USER: root
<xxx.compute-1.amazonaws.com> SSH: EXEC ssh -C -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=/root/.ansible/cp/%C xxx.compute-1.amazonaws.com '/bin/sh -c '"'"'/usr/bin/python && sleep 0'"'"''
<xxx.compute-1.amazonaws.com> (0, 'Please login as the user "ec2-user" rather than the user "root".\n\n', "Warning: Permanently added 'xxx.compute-1.amazonaws.com,...' (ECDSA) to the list of known hosts.\r\n")
fatal: [xxx.compute-1.amazonaws.com]: FAILED! => {
    "changed": false, 
    "failed": true, 
    "module_stderr": "Warning: Permanently added 'xxx.compute-1.amazonaws.com,...' (ECDSA) to the list of known hosts.\r\n", 
    "module_stdout": "Please login as the user \"ec2-user\" rather than the user \"root\".\n\n", 
    "msg": "MODULE FAILURE", 
    "rc": 0
}
	to retry, use: --limit @/usr/share/ansible/openshift-ansible/playbooks/aws/openshift-cluster/build_ami.retry

PLAY RECAP ***********************************************************************************************************************************************************************************
xxx.compute-1.amazonaws.com : ok=1    changed=0    unreachable=0    failed=1   
localhost                  : ok=21   changed=1    unreachable=0    failed=0   
...

Expected results:
Installer should use ec2-user.

Additional info:
# grep -nir "openshift_aws_build_ami_ssh_user" .
./playbooks/aws/openshift-cluster/build_ami.yml:37:      ansible_ssh_user: "{{ openshift_aws_build_ami_ssh_user | default('root') }}"