Convert poll_rescued_instances() periodic task to objects

This converts poll_rescued_instances() to use instance objects for
fetching the list of unrescued instances, and makes it call directly
to compute api instead of going through conductor. Now that
compute_api.unrescue() uses objects natively, we can avoid the
indirection through conductor, which makes it faster and reduces
messaging load.

Related to blueprint compute-manager-objects-juno

Change-Id: I68b2b57831048932277e1f4e388f8a1aa8309aff
Merge "Convert poll_rescued_instances() periodic task to objects"