Fix UT based on upstream change

neutron commit 9bf0e6654480f98f2315a43687b267263a82a823 removed
helper methdods from neutron.db.api.
Personal preference. This patch would have been a lot quicker if we had a base class like `BaseFunctionalTest` everywhere already. Similarly, if we have a base class in each file it would be easier to make sweeping per-file changes.