Update tests neutron.conf to use transport_url

The deprecated rpc_backend parameter was removed from oslo.messaging in
I193cc0e613459a6dbbfd54ed0901a54ded78d712

Use transport_url parameter instead

Change-Id: Ia73fe052986617dd6ce5e68cced0c4c88516a33e
Closes-Bug: #1795878

neutron/tests/base.py

@@ -291,7 +291,6 @@ class BaseTestCase(DietTestCase):
     @staticmethod
     def config_parse(conf=None, args=None):
         """Create the default configurations."""
-        # neutron.conf includes rpc_backend which needs to be cleaned up
         if args is None:
             args = []
         args += ['--config-file', etcdir('neutron.conf')]

neutron/tests/etc/neutron.conf

@@ -14,8 +14,8 @@ bind_port = 9696
 # Paste configuration file
 api_paste_config = api-paste.ini.test
 
-# The messaging module to use, defaults to kombu.
-rpc_backend = fake
+# The messaging transport URL
+transport_url = 'fake:/'
 
 lock_path = $state_path/lock