The db.api module provides a useless indirection to the only
implementation we ever had, sqlalchemy. Let's use that directly instead
of the wrapper.
Change-Id: I80353cfed801b95571523515fd3228eae45c96ae
1. don't return DB models from the objects API
(only return Objects)
2. delete shouldn't return anything
3. update_and_save should return the refreshed object.
Note: there is still some inconsistency in what is returned by
update_by_id() some return an object and some return a bool.
Related-bug: #1432936
Change-Id: I1a0a38773d4fc4a62af5e0a98076396f39187b6c
Thomas Herve