remove default=None for config options

In the cfg module default=None is set as the default value.

Change-Id: I5de0c1ddeeaccde8183ec130c85524f5b57f6f74
Closes-Bug: #1323975

oslo/messaging/_drivers/impl_zmq.py

@@ -69,7 +69,7 @@ zmq_opts = [
     cfg.IntOpt('rpc_zmq_contexts', default=1,
                help='Number of ZeroMQ contexts, defaults to 1.'),
 
-    cfg.IntOpt('rpc_zmq_topic_backlog', default=None,
+    cfg.IntOpt('rpc_zmq_topic_backlog',
                help='Maximum number of ingress messages to locally buffer '
                     'per topic. Default is unlimited.'),
 

oslo/messaging/_drivers/matchmaker_redis.py

@@ -32,7 +32,6 @@ matchmaker_redis_opts = [
                default=6379,
                help='Use this port to connect to redis host.'),
     cfg.StrOpt('password',
-               default=None,
                help='Password for Redis server (optional).'),
 ]
 

oslo/messaging/transport.py

@@ -37,7 +37,6 @@ from oslo.messaging import exceptions
 
 _transport_opts = [
     cfg.StrOpt('transport_url',
-               default=None,
                help='A URL representing the messaging driver to use and its '
                     'full configuration. If not set, we fall back to the '
                     'rpc_backend option and driver specific configuration.'),