Object-ify APIv3 flavor_extraspecs extension

This makes the APIv3 flavor_extraspecs extension use the Flavor
object instead of going direct to the database.

Related to blueprint compute-manager-objects-juno

Change-Id: Ibe9211fb944bb64f722e2417725e29506b17ee30
Merge "Object-ify APIv3 flavor_extraspecs extension"