Tech-debt: test_create of bay conductor need to be improve

[Eli Qiao]

This is from review:

https://review.openstack.org/#/c/257272/6/magnum/tests/unit/conductor/handlers/test_bay_conductor.py

       # FixMe(eliqiao): bay_create will call bay.create() again, this so bad
        # because we have already called it in setUp since other test case will
        # share the codes in setUp()
        # But in self.handler.bay_create, we update bay.uuid and bay.stack_id
        # so bay.create will create a new recored with baymodel_id None,
        # this is bad because we load BayModel object in Bay object by
        # baymodel_id. Here update self.bay.baymodel_id so bay.obj_get_changes
        # will get notice that baymodel_id is updated and will update it
        # in db.