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[1].
The overhead of creating a temporary list on Python 2 is negligible.
[1]http://lists.openstack.org/pipermail/openstack-dev/2015-June/066391.html
Partially-implements blueprint py3-compatibility
Change-Id: Ia2298733188b3d964d43a547504ede2ebeaba9bd