change the volume_support default to False

With volume_support enabled, trove will require that new instances use
swift. It works fine without swift though and I suppose we want to avoid
an unneeded dependency on swift.
So with the code as it currently is in this commit, I get the right message when trying to apply the proposal with trove_support enabled and no swift_instance. If I delete the `validate_proposal_after_save` method, the proposal never applies, because it always ends up in queued.