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.
Did you have a swift proposal when you created the proposal?

Also, that reminds me the check that I think I initially wanted to suggest:
```
if proposal["attributes"][@bc_name]["volume_support"] && proposal["attributes"][@bc_name]["swift_instance"] == "none"
  validation_error("No swift instance! Write better message")
end
```