Make resource tracker always use Flavor objects

Make sure that even when the Flavor is extracted from the instance by
_get_instance_type, it is always a Flavor object we can work with.

Also fix test_tracker.py tests to actually extract the flavor from the
instance, and not fall back on Flavor.get_by_id in
ResourceTracker,_get_instance_type, since we should really never hit
that under normal circumstances.

Change-Id: If704ac386bd4f35dbb7b8e5c46fdd240445ee0a4
Blueprint: virt-driver-cpu-pinning
Merge "Make resource tracker always use Flavor objects"