Deprecate the mandatory flag

It will not be possible to deactivate this functionality anymore.

Change-Id: I1cbafff03349f7da9224de46285707fbf2a81a68
(cherry picked from commit 2b89d97888)

oslo_messaging/_drivers/impl_rabbit.py

@@ -169,10 +169,14 @@ rabbit_opts = [
                'we check the heartbeat.'),
     cfg.BoolOpt('direct_mandatory_flag',
                 default=True,
-                help='Enable/Disable the RabbitMQ mandatory flag '
-                'for direct send. The direct send is used as reply, '
+                deprecated_for_removal=True,
+                deprecated_reason='Mandatory flag no longer deactivable.',
+                help='(DEPRECATED) Enable/Disable the RabbitMQ mandatory '
+                'flag for direct send. The direct send is used as reply, '
                 'so the MessageUndeliverable exception is raised '
-                'in case the client queue does not exist.'),
+                'in case the client queue does not exist.'
+                'This flag is deprecated and it will not be possible to '
+                'deactivate this functionality anymore'),
     cfg.BoolOpt('enable_cancel_on_failover',
                 default=False,
                 help="Enable x-cancel-on-ha-failover flag so that "

releasenotes/notes/disable-mandatory-flag-a6210a534f3853f0.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Deprecating the ``direct_mandatory_flag``. It will not be possible to
+    deactivate this functionality anymore.