Remove a TODO item

The node membership is always updated today when it has been changed.
This patch removes a TODO item related to this.

Change-Id: Ie0b48a0e5edd1d6adb5a42cd86ac25b6657481b1

senlin/engine/cluster.py

@@ -81,7 +81,6 @@ class Cluster(object):
         self.metadata = kwargs.get('metadata') or {}
 
         # rt is a dict for runtime data
-        # TODO(Qiming): nodes have to be reloaded when membership changes
         self.rt = {
             'profile': None,
             'nodes': [],