GlusterFS: Fix deadlock in volume clone

The create_cloned_volume path could deadlock due to
create_cloned_volume and create/delete_snapshot using the same
lock for synchronization.

Refactor the calls to create/delete snapshot to call the inner
method which does not use a lock.

Introduced by "06999f6 GlusterFS: Synchronize additional op..."

Related-Bug: 1267983
Closes-Bug: 1272092

Change-Id: I84ca34b201c10644faa047f1c9274c14bcdd0359
Merge "GlusterFS: Fix deadlock in volume clone"
(Test commit) Change README to reflect Jio's fork of Cinder

Change-Id: I3f26c0a736e63725aec24ca2fa12c707b8e6460f