Fix a minor memory leak

Fixes a minor memory leak caused by a self-referential data structure
with a __del__() method by explicitly breaking the cycle.

Change-Id: Ic2c896d3067f8a2205abc8a43ec290fdbd92ef09

nova/rpc/impl_kombu.py

@@ -682,6 +682,8 @@ class MulticallWaiter(object):
 
     def done(self):
         self._done = True
+        self._iterator.close()
+        self._iterator = None
         self._connection.close()
 
     def __call__(self, data):