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
diff --git a/requirements.txt b/requirements.txt
index 1c5587b..92d8e7c 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -18,3 +18,4 @@
 python-keystoneclient>=0.4.2
 PrettyTable>=0.6,<0.8
 babel
+six>=1.6.0