Validate NtpServers during plan processing
Validate NtpServers during plan processing

Moving this to an earlier function allows us to decouple the plan
processing from the heat_deploy which deploys the plan.  In future
it would be good to move this to a mistral validation instead of
a client-side special case.

Change-Id: If9e9fceb85408221cebbbacb8908f73b42268c19
Partial-Bug: #1635409

openstack/python-tripleoclient