Add support for swap_volume

Add client support for swap_volume, which allows swapping a volume
currently attached to an instance with a different volume which is not
attached. The contents of the old volume will be copied onto the new
volume. This was added in nova commit
8f51b120b430c7c21399256f37e1d8f75d030484.

Change-Id: I98323d594617c1c435e403d9f3ddc4ff4fa74da6
Merge "Add support for swap_volume"