Create sized volume floc 978

Fixes FLOC-978
It seems a bit funny to create a  `flocker.volume.Volume` here. I can see how @exarkun 's comment in https://github.com/ClusterHQ/flocker/commit/5459add51d209177b31be7b8120ce5aacc8dcc6a#diff-c0fb9628490399a6a6c1256796512853R123 might be interpreted that way, but I'd have interpreted the design to mean that `volume_service.create` should accept an `AttachedVolume` and that `create` would then be responsible for copying all the parameters to a new *real* volume.

In https://github.com/ClusterHQ/flocker/commit/5459add51d209177b31be7b8120ce5aacc8dcc6a#diff-c0fb9628490399a6a6c1256796512853R123 @exarkun wrote:
> add maximum_size parameter or better, replace name with a volume model object that can describe many parameters
> initialize Volume with maximum_size

And then maybe `_to_volume_name` can be moved into `flocker.volume.???`, here or in a followup branch.