required conditionals should be verified before proceeding

[thoraxe]

I got kinda deep into an install and then hit the following:

TASK [nfs : install nfs packages] **********************************************
fatal: [ip-192-199-0-201.ec2.internal]: FAILED! => {"failed": true, "msg": "The conditional check '{{ install_nfs }}' failed. The error was: error while evaluating conditional ({{ install_nfs }}): 'install_nfs' is undefined"}
        to retry, use: --limit @/home/thoraxe/Red_Hat/openshift/ansible_aws_deployer/ansible/main.retry

It would have made sense to do some kind of pre-flight check for that.