Stub out strip_node for DeletedResource

This is only ever a URL at the moment.  Do not trigger stack
traces in firmware update by stubbing out the strip_node.

confluent_server/confluent/messages.py

@@ -246,6 +246,9 @@ class DeletedResource(ConfluentMessage):
     def __init__(self, resource):
         self.kvpairs = {'deleted': resource}
 
+    def strip_node(self, node):
+        pass
+
 class CreatedResource(ConfluentMessage):
     notnode = True
     readonly = True