RemoteFsDriver: copy_image_to_volume and copy_volume_to_image

This functionality was recently added to the NFS driver --
it should instead be added to the RemoteFsDriver class to cover
both NFS and GlusterFS drivers.

Change-Id: I10136f8d1628128a93c4adfe2cb9447b99720558
Merge "RemoteFsDriver: copy_image_to_volume and copy_volume_to_image"