Fix cross-import bug in cinder.db.sqlalchemy.api

Cross import in cinder.sqlalchemy.db.api module breaks
if lazy loading of db backend is removed

In oslo.incubator db.api lazy loading was removed
Check this change for details https://review.openstack.org/#/c/55985/

Change-Id: I6a75afd0b9b55097ef76ad24c53e84339b527196
Merge "Fix cross-import bug in cinder.db.sqlalchemy.api"