Run test_db_api on all backends (sqlite, mysql, postgresql) for OpenStack Compute (nova)

There is a lot of different things in SQl backends. For example work with casting. In current SQLite we are able to store everything in column (with any type). Mysql will try to convert value to required type, and postgresql will raise IntegrityError. So to avoid such nasty errors on db.api layer we should runt test...