Only load timestamp service plugin in timestamp tests

Do not allow the timestamp plugin tests to load all of the default
service plugins to prevent possible resources from being leaked
from those plugins.

Change-Id: Ic31cc86efe8b7415e2a4769d5c2f542d1e7097f5
Closes-Bug: #1569130

neutron/tests/unit/extensions/test_timestamp_core.py

@@ -16,7 +16,6 @@ import datetime
 import six
 
 import mock
-from oslo_config import cfg
 
 from neutron import context
 from neutron.db import db_base_plugin_v2
@@ -61,7 +60,9 @@ class TimeStampChangedsinceTestCase(test_db_base_plugin_v2.
         self.addCleanup(manager.NeutronManager.clear_instance)
 
     def setup_coreplugin(self, core_plugin=None):
-        cfg.CONF.set_override('core_plugin', self.plugin)
+        super(TimeStampChangedsinceTestCase, self).setup_coreplugin(
+            self.plugin)
+        self.patched_default_svc_plugins.return_value = ['timestamp_core']
 
     def _get_resp_with_changed_since(self, resource_type, changed_since):
         query_params = 'changed_since=%s' % changed_since