Fix oslo.messaging default transport

Change d3f6ca0b47 have broken
oslo.messaging defaults, no more transport are selected by default and
oslo.messaging just crash:

    Traceback (most recent call last):
      File "/home/sileht/workspace/openstack/oslo.messaging/.tox/py27/lib/python2.7/site-packages/mock/mock.py", line 1305, in patched
        return func(*args, **keywargs)
      File "oslo_messaging/tests/test_transport.py", line 111, in test_get_transport
        transport_ = oslo_messaging.get_transport(self.conf, **kwargs)
      File "/home/sileht/workspace/openstack/oslo.messaging/.tox/py27/lib/python2.7/site-packages/debtcollector/removals.py", line 242, in wrapper
        return f(*args, **kwargs)
      File "oslo_messaging/transport.py", line 247, in get_transport
        transport_cls=RPCTransport)
      File "oslo_messaging/transport.py", line 205, in _get_transport
        url.transport.split('+')[0],
    AttributeError: 'NoneType' object has no attribute 'split'

This change restores rabbit:// as default.

Change-Id: I945a2aca60347dad672cc406cd33e80e93c588a7

oslo_messaging/tests/test_transport.py

@@ -51,6 +51,13 @@ class _FakeManager(object):
 class GetTransportTestCase(test_utils.BaseTestCase):
 
     scenarios = [
+        ('default',
+         dict(url=None, transport_url=None,
+              control_exchange=None, allowed=None,
+              expect=dict(backend='rabbit',
+                          exchange=None,
+                          url='rabbit:',
+                          allowed=[]))),
         ('transport_url',
          dict(url=None, transport_url='testtransport:',
               control_exchange=None, allowed=None,
@@ -83,8 +90,10 @@ class GetTransportTestCase(test_utils.BaseTestCase):
 
     @mock.patch('oslo_messaging.transport.LOG')
     def test_get_transport(self, fake_logger):
-        self.config(control_exchange=self.control_exchange,
+        self.messaging_conf.reset()
-                    transport_url=self.transport_url)
+        self.config(control_exchange=self.control_exchange)
+        if self.transport_url:
+            self.config(transport_url=self.transport_url)
 
         driver.DriverManager = mock.Mock()
 

oslo_messaging/transport.py

@@ -42,6 +42,7 @@ LOG = logging.getLogger(__name__)
 
 _transport_opts = [
     cfg.StrOpt('transport_url',
+               default="rabbit://",
                secret=True,
                help='The network address and optional user credentials for '
                     'connecting to the messaging backend, in URL format. The '
@@ -443,8 +444,6 @@ class TransportURL(object):
         if not url:
             conf.register_opts(_transport_opts)
         url = url or conf.transport_url
-        if not url:
-            return cls(conf)
 
         if not isinstance(url, six.string_types):
             raise InvalidTransportURL(url, 'Wrong URL type')