py3:Remove six.iterXXX

1、As mentioned in [1], we should avoid using six.iteritems/keys
   achieve iterators. We can use dict.items/keys 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

TrivialFix.

Change-Id: I0cbe8af3210233a58d25f0df187c3d085405aa2a