Replace data structures' attribute with six module

In Python 3, some data structures' attribute is different in Python 2.
See http://pythonhosted.org/six/#object-model-compatibility
This is change mapping:

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

Implements: blueprint make-python3-compatible
Change-Id: Ida48f39ff230860feee7305b93b134c625a21663