core (memory/disco): fix hard encoded value in findServiceEntity

sat/memory/disco.py

@@ -303,7 +303,7 @@ class Discovery(object):
         args are the same as for [findServiceEntities]
         @return (jid.JID, None): found entity
         """
-        d = self.host.findServiceEntities(client, "pubsub", "service")
+        d = self.host.findServiceEntities(client, category, type_)
         d.addCallback(lambda entities: entities.pop() if entities else None)
         return d