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. Removed six from test requirements.

Change-Id: Ifba282b1bde9681268e6e46ffd024c96c6b9be05
Merge "replace dict.iteritems() with six.iteritems(dict)"