Add flavor access methods to Instance object

This adds a native interface to the Instance object to get, save, and
delete either the main or stashed flavors. This will let us migrate
code away from flavors.py and also provide us a versioned method to
handle conversion of flavors, flavor extra specs, and system_metadata
to the JSON blob that we have planned.

Related to blueprint compute-manager-objects

Change-Id: I13a4903460b05798e9c95b2148fe93e4963d6361
Merge "Add flavor access methods to Instance object"