Volume cloning - design sketch for 739

Design sketch for #739; builds on #771... only relevant parts are in https://github.com/ClusterHQ/flocker/commit/ca2a73355204e6d9371fc6470e8a3c925b048434
Perhaps making this `clone_to` (or `clone_from` with the existing order) would make it easier to correctly guess/remember the order of arguments.