UT: Fix "RuntimeError: stop called on unstarted patcher"

By removing explicit addCleanup calls.
and also change import '_' from neutronclient.i18n import _
to networking_l2gw._i18n import _.

Change-Id: I2001c0d0b00410188942c02f7b8b43d3d2fd5b22
Closes-Bug: 1608389

networking_l2gw/l2gatewayclient/l2gw_client_ext/_l2_gateway.py

@@ -16,9 +16,10 @@
 
 from neutronclient.common import extension
 from neutronclient.common import utils
-from neutronclient.i18n import _
 from oslo_serialization import jsonutils
 
+from networking_l2gw._i18n import _
+
 INTERFACE_DELIMITER = ";"
 SEGMENTATION_ID_DELIMITER = "#"
 INTERFACE_SEG_ID_DELIMITER = "|"

networking_l2gw/l2gatewayclient/l2gw_client_ext/_l2_gateway_connection.py

@@ -15,9 +15,10 @@
 #
 
 from neutronclient.common import extension
-from neutronclient.i18n import _
 from neutronclient.neutron import v2_0 as l2gatewayV20
 
+from networking_l2gw._i18n import _
+
 
 class L2GatewayConnection(extension.NeutronClientExtension):
     resource = 'l2_gateway_connection'

networking_l2gw/tests/unit/l2gatewayclient/test_cli20_l2gw_conn_ext.py

@@ -34,7 +34,6 @@ class CLITestV20ExtensionL2GWJSON(test_cli20.CLITestV20Base):
     def _create_patch(self, name, func=None):
         patcher = mock.patch(name)
         thing = patcher.start()
-        self.addCleanup(patcher.stop)
         return thing
 
     def _mock_extension_loading(self):

networking_l2gw/tests/unit/l2gatewayclient/test_cli20_l2gw_ext.py

@@ -34,7 +34,6 @@ class CLITestV20ExtensionL2GWJSON(test_cli20.CLITestV20Base):
     def _create_patch(self, name, func=None):
         patcher = mock.patch(name)
         thing = patcher.start()
-        self.addCleanup(patcher.stop)
         return thing
 
     def _mock_extension_loading(self):