Update db module from oslo-incubator

Update to sha 1ebf819e31dde74f4374c26a791458ccd25b29d9

Note:
 - database.backend is now defined in solum/openstack/common/db/api.py
 - Added the migration_cli for alembic

Change-Id: Ida884b1b72b4045e3c42c6b90ac0279ce0e19725
Merge "Update db module from oslo-incubator"