use correct manager for volume snapshots

the snapshot manager is volume_snapshots, not snapshots.

Closes-Bug: #1567895
Change-Id: I12c0238fc04507c97e92089fda3bec816171a973

Author: Steve Martinelli

openstackclient/volume/v2/volume.py

@@ -110,7 +110,7 @@ def take_action(self, parsed_args):
         snapshot = None
         if parsed_args.snapshot:
             snapshot = utils.find_resource(
-                volume_client.snapshots,
+                volume_client.volume_snapshots,
                 parsed_args.snapshot).id
 
         project = None