Move helper DB functions to db.sqlalchemy.utils

Database utilities are contained in
openstack/common/db/sqlalchemy/test_migrations.py which is not
appropriate to the module appointment. All the utility functions moved
to openstack/common/db/sqlalchemy/utils.py.

Change-Id: Ia5e1efd3f8e2e1f9553ca24ca7bb19bea911cdbc
Merge "Move helper DB functions to db.sqlalchemy.utils"