Replace six.iteritems() with .items()

We should avoid using six.iteritems/keys achieve iterators. We can use
dict.items/keys instead, as it will return iterators in PY3 as well.
And dict.items/keys will more readable.

In py2, the performance about list should be negligible, see
https://wiki.openstack.org/wiki/Python3 and
http://lists.openstack.org/pipermail/openstack-dev/2015-June/066391.html

Change-Id: I6353f0a1b423c6dbd0c71343f7919fd0de440e34

elements/svc-map/extra-data.d/10-merge-svc-map-files

@@ -15,7 +15,6 @@
 import os
 import sys
 
-import six
 import yaml
 
 
@@ -39,7 +38,7 @@ def merge_data(source, destination, distro):
     result = dict()
     result.update(destination)
     family = os_family(distro)
-    for servicename, mapping in six.iteritems(source):
+    for servicename, mapping in source.items():
         if servicename in result:
             raise Exception("%s already found in services list" % servicename)
         if distro in mapping: