Allow snapshot_delete for NFS/GlusterFS drivers

RemoteFsDriver should allow snapshot_delete, since snapshot_create
for NFS and GlusterFS volumes will create a snapshot in the error
state which cannot be deleted afterward.

snapshot_delete() should allow this to be removed rather than
throwing NotImplementedError.

Fixes bug: 1156686

Change-Id: I4307878fa97fbce5bd733a83fa172509e2fba8ce
Merge "Allow snapshot_delete for NFS/GlusterFS drivers"