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.
> No. I did not have a swift proposal when I created this proposal. This looks like a barclamp-crowbar bug.

Then it's really why you see this error. The `*_instance` attributes are really only set on creation, so you should edit the raw attributes and change it.

> I don't see how that second test is better than the one I have proposed here. Or did you mean to add it in a different place?

It is better because on save, it will tell you that you want volume support but the attribute to reference the swift proposal is not set. So you will avoir the issue you mentioned above and get an error message instead.