Replace six.iteritems() with .items()

1.As mentioned in [1], we should avoid usingg
six.iteritems to achieve iterators. We can
use dict.items instead, as it will return
iterators in PY3 as well. And dict.items/keys
will more readable. 2.In py2, the performance
about list should be negligible, see the link [2].
[1] https://wiki.openstack.org/wiki/Python3
[2] http://lists.openstack.org/pipermail/openstack-dev/2015-June/066391.html

Change-Id: I71c13040318eca6e5ed993e8aa03f8003986a71c

glanceclient/v2/images.py

@@ -146,7 +146,7 @@ class Controller(object):
 
             tags_url_params.append({'tag': encodeutils.safe_encode(tag)})
 
-        for param, value in six.iteritems(filters):
+        for param, value in filters.items():
             if isinstance(value, six.string_types):
                 filters[param] = encodeutils.safe_encode(value)