[2/3]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

The patch list:
    1. cells.
    2. compute api.
    3. image.
    4. network.
    5. objects.
    6. scheduler.
    7. virt.
    8. other resources.

Partial-Implements: blueprint replace-iteritems-with-items

Change-Id: Ic6e469eb80ee1774de1374bb36f38b5134b6b311

nova/exception_wrapper.py

@@ -14,7 +14,7 @@ import functools
 import inspect
 
 from oslo_utils import excutils
-import six
+
 
 import nova.conf
 from nova.notifications.objects import base
@@ -91,4 +91,4 @@ def _get_call_dict(function, self, context, *args, **kw):
 
 def _cleanse_dict(original):
     """Strip all admin_password, new_pass, rescue_pass keys from a dict."""
-    return {k: v for k, v in six.iteritems(original) if "_pass" not in k}
+    return {k: v for k, v in original.items() if "_pass" not in k}