replace dict.iteritems() with six.iteritems(dict)

According to https://wiki.openstack.org/wiki/Python3 dict.iteritems()
should be replaced with six.iteritems(dict).

Added six >= 1.6.0 as new requirement.

Change-Id: Ie8a3eacc00dd9248ee80956b3e72f0e80655b1f1