Wait for hypervisor-stats to reflect ironic nodes

When enrolling nodes into Ironic, poll nova's hypervisor-stats until
the hypervisor count reflects the number of enrolled nodes.  This
eliminates a race where devstack completes and an instance is spawned
before the first post-enrollment periodic task ticks on the Nova side,
which has recently started popping up in the gate.

Change-Id: Ib3d8005e0094ee8af2d5fcb65aca6cd92736da90
Closes-bug: #1398128
Merge "Wait for hypervisor-stats to reflect ironic nodes"