Ensure return for db api functions

Functions in db/api.py should return whatever the implementation of
those functions return. This is important for qos_specs_update which
returns something for the sqlalchemy implementation, but for others as
well for future implementations.

Added 'return' for functions that were missing it.

Change-Id: I384b1cf915b2d2238fd75d23b17558cae2cf9f50
Closes-Bug: #1272365
Merge "Ensure return for db api functions"