Fix nova-bm race condition with cached images.

devtest can fail due to attempting to bring up heat stack before
baremetal nodes are available due to the latency between node
registration and exporting to the scheduler.

This is an alternative implementation to
I720ebf1333607b0daa2f8989f7e31d02f7903ef3.

Closes-Bug: #1288199
Change-Id: I3fca328a2fddf36106a1a17aacaa13e39a45b8a0
Co-Authored-By: Robert Collins <rbtcollins@hp.com>
Merge "Fix nova-bm race condition with cached images."