Create sized volume floc 978

Fixes FLOC-978
The points about `AttachedVolume` not being the right model type to use here are just right, I think.  We **need** a proper volume model object but we don't have one yet.  `AttachedVolume` should really be considered a link between an application and a volume, not a first class representation of a volume itself.  I think `create(get(several parameters))` is the solution that makes the most sense for now (but introducing a proper volume model object should let us improve this).