Disable and mark heartbeat as experimental

Due to some discovered issues since heartbeat is enabled by default.
Specially #1436788, that needs to fix the underlying library, too.
So, according to the discution here:
https://bugs.launchpad.net/oslo.messaging/+bug/1436769/comments/10

We decide to mark the implementation as experimental and disable it by default.

Related-bug: #1436788
Related-bug: #1436769
Change-Id: Ib7c55977f976bdbbc8df4ad5915e0433cbf84a17

oslo_messaging/_drivers/impl_rabbit.py

@@ -123,10 +123,10 @@
                      'If you change this option, you must wipe the '
                      'RabbitMQ database.'),
     cfg.IntOpt('heartbeat_timeout_threshold',
-               default=60,
+               default=0,
                help="Number of seconds after which the Rabbit broker is "
                "considered down if heartbeat's keep-alive fails "
-               "(0 disable the heartbeat)."),
+               "(0 disable the heartbeat). EXPERIMENTAL"),
     cfg.IntOpt('heartbeat_rate',
                default=2,
                help='How often times during the heartbeat_timeout_threshold '

oslo_messaging/tests/drivers/test_impl_rabbit.py

@@ -58,6 +58,12 @@ def test_driver_load(self):
 
 class TestHeartbeat(test_utils.BaseTestCase):
 
+    def setUp(self):
+        super(TestHeartbeat, self).setUp(
+            conf=cfg.ConfigOpts())
+        self.config(heartbeat_timeout_threshold=60,
+                    group='oslo_messaging_rabbit')
+
     @mock.patch('oslo_messaging._drivers.impl_rabbit.LOG')
     @mock.patch('kombu.connection.Connection.heartbeat_check')
     @mock.patch('oslo_messaging._drivers.impl_rabbit.Connection.'
@@ -164,7 +170,7 @@ def test_driver_load(self, connection_klass, fake_ensure):
         transport._driver._get_connection()
         connection_klass.assert_called_once_with(
             'memory:///', ssl=self.expected, login_method='AMQPLAIN',
-            heartbeat=60, failover_strategy="shuffle")
+            heartbeat=0, failover_strategy="shuffle")
 
 
 class TestRabbitIterconsume(test_utils.BaseTestCase):