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

cinder/volume/drivers/nfs.py

@@ -66,6 +66,11 @@ class RemoteFsDriver(driver.VolumeDriver):
     def delete_volume(self, volume):
         raise NotImplementedError()
 
+    def delete_snapshot(self, snapshot):
+        """Do nothing for this driver, but allow manager to handle deletion
+           of snapshot in error state."""
+        pass
+
     def ensure_export(self, ctx, volume):
         raise NotImplementedError()