Stub out attach_volume() in test_api_samples

If you enable CastToCall, the attach_volume_to_server() test fails when
it attempts to connect to cinder (i.e. initialize_connection()).

Best thing is to just stub out the attach_volume() call.

blueprint: oslo-messaging
Change-Id: I75f4e3fd1ecd0635867c167872a8eb7606cf295e
Merge "Stub out attach_volume() in test_api_samples"