Warn when we force creating a non durable exchange

Adding warning logs so that users can detect the fallback with durable
exchanges.

Change-Id: Iabce0986fae6ed8838f1f94496b5994fc19cc5ef

oslo_messaging/_drivers/impl_rabbit.py

@@ -1274,6 +1274,7 @@ class Connection(object):
                 # from the one used first.
                 if "PRECONDITION_FAILED - inequivalent arg 'durable'" \
                    in str(err):
+                    LOG.warning("Force creating a non durable exchange.")
                     exchange.durable = False
                     exchange(self.channel).declare()
             self._declared_exchanges.add(exchange.name)