Register all sqlalchemy events through a new function in
neutron.db.api so we can keep track of active events and
ensure all are removed at the end of each test run.
Without this, an instance of a plugin may be left around
with the only reference to it existing in SQLAlchemy, where
it will receive events for tests unrelated to it and potentially
interfere.
Change-Id: I8e93eb4e8ef5a13f015db9cd20e44941cdcb72ef
553ab6d86e