Discard reference when no longer needed

If an 'onlogon' handler is passed, discard it after being invoked.  This allows
the objects to be garbage collected where otherwise they may not be.

Change-Id: I92488adca9150204512db5e4872e2b54bd0c076d

pyghmi/ipmi/command.py

@@ -147,6 +147,7 @@ class Command(object):
 
     def logged(self, response):
         self.onlogon(response, self)
+        self.onlogon = None
 
     @classmethod
     def eventloop(cls):