Create RemoteFSSnapDriver class

Move RemoteFSDriver into its own file.

Refactor pieces needed for qcow2 snapshots into a class.

Also add self.SHARE_FORMAT_REGEX.

Implements bp: remotefs-snaps

Change-Id: I8f9eaed125f4b916b835a63b04239058e34492a9
Merge "Create RemoteFSSnapDriver class"