Use messaging notifications transport instead of default

The usage of oslo_messaging.get_transport is not meant for
notifications; And oslo_messaging.get_notification_transport is
favored for those means. So this change introduces the usage of that
function.

If the settings for the notifications are not set with the
configuration that's under the oslo_messaging_notifications group,
this will fall back to the old settings which are under the DEFAULT
group; just like this used to work.

Change-Id: Ic72fd658fe9b35d39cc3d7da2e4b9fe3454edfba

ironic/common/rpc.py

@@ -22,6 +22,7 @@ from ironic.common import exception
 
 CONF = cfg.CONF
 TRANSPORT = None
+NOTIFICATION_TRANSPORT = None
 NOTIFIER = None
 
 ALLOWED_EXMODS = [
@@ -47,16 +48,23 @@ def init(conf):
     TRANSPORT = messaging.get_transport(conf,
                                         allowed_remote_exmods=exmods,
                                         aliases=TRANSPORT_ALIASES)
+    NOTIFICATION_TRANSPORT = messaging.get_notification_transport(
+        conf,
+        allowed_remote_exmods=exmods,
+        aliases=TRANSPORT_ALIASES)
     serializer = RequestContextSerializer(messaging.JsonPayloadSerializer())
-    NOTIFIER = messaging.Notifier(TRANSPORT, serializer=serializer)
+    NOTIFIER = messaging.Notifier(NOTIFICATION_TRANSPORT,
+                                  serializer=serializer)
 
 
 def cleanup():
-    global TRANSPORT, NOTIFIER
+    global TRANSPORT, NOTIFICATION_TRANSPORT, NOTIFIER
     assert TRANSPORT is not None
+    assert NOTIFICATION_TRANSPORT is not None
     assert NOTIFIER is not None
     TRANSPORT.cleanup()
-    TRANSPORT = NOTIFIER = None
+    NOTIFICATION_TRANSPORT.cleanup()
+    TRANSPORT = NOTIFICATION_TRANSPORT = NOTIFIER = None
 
 
 def set_defaults(control_exchange):

releasenotes/notes/configure-notifications-72824356e7d8832a.yaml

@@ -0,0 +1,8 @@
+---
+features:
+  - It is now possible to configure the notifications tu use a different
+    transport URL than the RPCs. These could potentially be completely
+    different message broker hosts (though they don't need to be). If the
+    notification-specific configuration is not provided, the notifier will use
+    the same transport as the RPCs.
+