Replace 'import json' with oslo_serialization

Replace remaining occurences of 'import json' with
'from oslo_serialization import jsonutils as json'
so pylint doesn't complain every time someone happens
to make a change to one of the modules that still
uses it.

Change-Id: Ife9f0fc54ad36887bdb939028f8903be16e590d6
diff --git a/neutron/tests/tempest/test.py b/neutron/tests/tempest/test.py
index d95174b..3abf826 100644
--- a/neutron/tests/tempest/test.py
+++ b/neutron/tests/tempest/test.py
@@ -15,7 +15,6 @@
 
 import atexit
 import functools
-import json
 import os
 import re
 import sys
@@ -24,6 +23,7 @@
 
 import fixtures
 from oslo_log import log as logging
+from oslo_serialization import jsonutils as json
 from oslo_utils import importutils
 import six
 from six.moves.urllib import parse