Replace six iteration methods with standard ones

1.As mentioned in [1], we should avoid using six.iterXXX
to achieve iterators. We can use dict.XXX instead, as it will
return iterators in PY3 as well.

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: I801290be0a2afa929a657821c419f935a908c5b4
diff --git a/ironic_tempest_plugin/services/baremetal/base.py b/ironic_tempest_plugin/services/baremetal/base.py
index 368df47..1b5c524 100644
--- a/ironic_tempest_plugin/services/baremetal/base.py
+++ b/ironic_tempest_plugin/services/baremetal/base.py
@@ -13,7 +13,6 @@
 import functools
 
 from oslo_serialization import jsonutils as json
-import six
 from six.moves.urllib import parse as urllib
 from tempest.lib.common import api_version_utils
 from tempest.lib.common import rest_client
@@ -98,7 +97,7 @@
 
         """
         def get_change(kwargs, path='/'):
-            for name, value in six.iteritems(kwargs):
+            for name, value in kwargs.items():
                 if isinstance(value, dict):
                     for ch in get_change(value, path + '%s/' % name):
                         yield ch