This is needed for objects that have fields that have different names
comparing to corresponding attributes in the database. Updated test
cases to reflect that behaviour (pass field filters into object
get_object(s), but expect db_api functions to receive and return model
attributes).
Partially-Implements: blueprint adopt-oslo-versioned-objects-for-db
Change-Id: Ib65922eeaa9573312e3428c748ff415ef600052c