Run rabbitmq heartbeat in python thread by default

Removing the experimental nature of this feature and activating it by default.
Now to run heartbeat in a green thread users should set this option to False.

Also deprecating this option to prepare future removal and force to always run
heartbeat in a python thread whatever the context.

Change-Id: I32a6c4ad0a456282ec02b5e4c8309489b3c17553

oslo_messaging/_drivers/impl_rabbit.py

@@ -88,14 +88,16 @@
                help='SSL certification authority file '
                     '(valid only if SSL enabled).'),
     cfg.BoolOpt('heartbeat_in_pthread',
-                default=False,
-                help="EXPERIMENTAL: Run the health check heartbeat thread "
-                     "through a native python thread. By default if this "
-                     "option isn't provided the  health check heartbeat will "
-                     "inherit the execution model from the parent process. By "
-                     "example if the parent process have monkey patched the "
+                default=True,
+                help="Run the health check heartbeat thread "
+                     "through a native python thread by default. If this "
+                     "option is equal to False then the health check "
+                     "heartbeat will inherit the execution model "
+                     "from the parent process. For "
+                     "example if the parent process has monkey patched the "
                      "stdlib by using eventlet/greenlet then the heartbeat "
-                     "will be run through a green thread."),
+                     "will be run through a green thread.",
+                deprecated_for_removal=True),
     cfg.FloatOpt('kombu_reconnect_delay',
                  default=1.0,
                  deprecated_group='DEFAULT',

releasenotes/notes/run-heartbeat-in-pthread-by-default-28637b41ebf500dc.yaml

@@ -0,0 +1,10 @@
+---
+upgrade:
+  - |
+    The ``[oslo_messaging_rabbit] heartbeat_in_pthread`` config option now
+    defaults to ``True``.
+    Applications will run RabbitMQ heartbeat in a Python thread by default.
+deprecations:
+  - |
+    ``heartbeat_in_pthread`` has been deprecated and will be removed in a
+    future release. If configured, this option should be unset.