TrivialFix: Replace six.iteritems() with .items()

1.As mentioned in [1], we should avoid using 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: I2a201cce19b66d61924738fb89ded7e265b780ec

zun/db/etcd/api.py

@@ -114,7 +114,7 @@ class EtcdAPI(object):
 
     def _filter_resources(self, resources, filters):
         for c in list(resources):
-            for k, v in six.iteritems(filters):
+            for k, v in filters.items():
                 if c.get(k) != v:
                     resources.remove(c)
                     break

zun/db/etcd/models.py

@@ -17,7 +17,6 @@ etcd models
 """
 
 import etcd
-import six
 
 from zun.common import exception
 import zun.db.etcd as db
@@ -55,7 +54,7 @@ class Base(object):
 
     def update(self, values):
         """Make the model object behave like a dict."""
-        for k, v in six.iteritems(values):
+        for k, v in values.items():
             setattr(self, k, v)
 
     def save(self, session=None):