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.
You also need to change the swift line above this line to be optional, with something like this:
```ruby
 if role.default_attributes[@bc_name]["volume_support"]
  answer << { "barclamp" => "swift", "inst" => role.default_attributes[@bc_name]["swift_instance"] }
end
```