Use six for iter keys/values

Both iter keys and values have different names in Python 2 and Python3.
See http://pythonhosted.org/six/#object-model-compatibility
Mapping for iterkeys() and itervalues():

   six               Python 2           Python 3
   six.iterkeys(dict)   dict.iterkeys()    dict.keys()
   six.itervalues(dict) dict.itervalues()  dict.values()

Implemenets blueprint make-py3x-compatible

Change-Id: Iddfdb5a04be0416f8678bfa1ee96f68f88d41a78