Use six to make dict work in Python 2 and Python 3

Dict's interface changed a lot between Python 2 and Python 3.
Use six to make dict work in them.
This is mapping:

six            Python 2        Python 3
six.iterkeys   dict.iterkeys   dict.keys
six.itervalues dict.itervalues dict.values
six.iteritems  dict.iteritems  dict.items

Part of blueprint keystone-py3kcompat

Change-Id: If41b19353f07853e39064f848ba74e9cdfd9bd14
Merge "Use six to make dict work in Python 2 and Python 3"