Re-add policy check for get_snapshot

This was mistakenly removed in the objects patch.

Change-Id: If3c1565d9d17846ab11dcb375c79e99006633c80
Closes-Bug: #1559250
(cherry picked from commit 35ffa62113)

cinder/volume/api.py

@@ -505,6 +505,7 @@ class API(base.Base):
         return volumes
 
     def get_snapshot(self, context, snapshot_id):
+        check_policy(context, 'get_snapshot')
         snapshot = objects.Snapshot.get_by_id(context, snapshot_id)
 
         # FIXME(jdg): The objects don't have the db name entries