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: Ia235afc3532f62f265f91ca46d2306c72fc2a2a2