Switch all uses of iteritems to use six instead

This commit switches all of the uses of iteritems in tempest to use
six.iteritems() instead so it can be run using python2 or python3.

Change-Id: Ice006bc5238edb072a9556559b467804d4604d49
diff --git a/tempest/cmd/javelin.py b/tempest/cmd/javelin.py
index b2311b0..4e2af76 100755
--- a/tempest/cmd/javelin.py
+++ b/tempest/cmd/javelin.py
@@ -114,6 +114,7 @@
 import netaddr
 from oslo_log import log as logging
 from oslo_utils import timeutils
+import six
 from tempest_lib import auth
 from tempest_lib import exceptions as lib_exc
 import yaml
@@ -416,7 +417,7 @@
         that things like tenantId didn't drift across versions.
         """
         LOG.info("checking users")
-        for name, user in self.users.iteritems():
+        for name, user in six.iteritems(self.users):
             client = keystone_admin()
             found = client.identity.get_user(user['id'])
             self.assertEqual(found['name'], user['name'])