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.

Conflicts:
	lib/ironic

Change-Id: Ib3d8005e0094ee8af2d5fcb65aca6cd92736da90
Closes-bug: #1398128
(cherry picked from commit c78e4671098f08c2eaf0b033777d3c01082141cd)
Merge "Wait for hypervisor-stats to reflect ironic nodes" into stable/juno
Merge branch 'stable/juno' of https://github.com/openstack-dev/devstack into stable/juno