new method
other fixes
GlusterFS: Copy snap from correct source file

The GlusterFS driver's copy_volume_from_snapshot was previously
not parsing the snapshot information to determine which backing
file to copy from, instead copying from the active file at all
times.  This would result in the wrong data being supplied when
a volume is cloned from a snapshot.

Add a test for the copy_volume_from_snapshot method.

Closes-Bug: #1222907
Change-Id: Ib829ca1a6812b61845f3b2eb9c5507779fa6ec15
newer fix