RemoteFS revert snapshot support

The goal of this blueprint is adding RemoteFS driver support for reverting snapshots . Note that we only consider the case in which the latest snapshot is being reverted (the Cinder API won't allow reverting volumes to other snapshots anyway, at least for the time being).

The RemoteFS based drivers (e.g. NFS driver, SMB driver) use COW images (e.g. qcow2, differencing vhdx) in order to provide volume snapshot functionality. Reverting a volume to its latest snapshot is a trivial operation in this case and can be achived by simply cleaning up (e.g. recreating) the latest image from chain. We intend to implement this simple change at the RemoteFS class level so that any drivers inheriting it can benefit from this feature.

Note that (efficiently) reverting a volume to a snapshot other than the last one may be more difficult to implement as the drivers expect to deal with a chain (list) of images, while in this case we may end up with a tree structure. As previously mentioned, this is outside the scope of this blueprint.