pep8: Fix E711, comparisons to None should use 'is' or 'is not'

Lengthy discussion of why this is a good thing (I didn't know) at SO:

http://stackoverflow.com/questions/2209755/python-operation-vs-is-not

mininet/log.py

@@ -117,7 +117,7 @@ class MininetLogger( Logger, object ):
            Convenience function to support lowercase names.
            levelName: level name from LEVELS"""
         level = LOGLEVELDEFAULT
-        if levelname != None:
+        if levelname is not None:
             if levelname not in LEVELS:
                 raise Exception( 'unknown levelname seen in setLogLevel' )
             else: