Wait for enough hosts to be available in nova

There is a race condition waiting for "!= 0" because it is entirely
possible nova refreshed the compute hosts list in between baremetal node
registrations, leading to a number != 0, but also not high enough to
create the overcloud.

Change-Id: Idd83aaaf0cb46d0f748e500771112698f9f50e65
Merge "Wait for enough hosts to be available in nova"