Replace six iteration methods with standard ones

1.As mentioned in [1], we should avoid using six.iterXXX
to achieve iterators. We can use dict.XXX instead, as it will
return iterators in PY3 as well.

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: I801290be0a2afa929a657821c419f935a908c5b4
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