Replace six.iteritems() with .items()
Replace six.iteritems() with .items()

1.As mentioned in [1], we should avoid using six.iteritems to achieve
iterators. We can use dict.items instead, as it will return iterators
in PY3 as well. And dict.items/keys will more readable.
2.In py2, the performance about list should be negligible, see the
link [2].
[1] https://wiki.openstack.org/wiki/Python3
[2] http://lists.openstack.org/pipermail/openstack-dev/2015-June/066391.html

Change-Id: Ib27b2784c6cfaa00c76a53b3c041c74866bbd66a

openstack/vitrage,openstack/vitrage,openstack/vitrage