Use dict.items() for adding Python 3 compatibility

Replacing dict.iteritems()/.itervalues() with
six.iteritems(dict)/six.itervalues(dict) was preferred in the past,
but there was a discussion suggesting to avoid six for this.

ref:
http://lists.openstack.org/pipermail/openstack-dev/2015-June/066391.html

Partially implements: blueprint python3
Change-Id: Ie931bc02f558f6db3677b44f645eb3bdf78ccae8