Enable sending events to notification bus by default

According to [1] in order to Aodh event alarms work notifier publisher
should be set with special topic (alarm.all by default).

[1] https://docs.openstack.org/aodh/latest/contributor/event-alarm.html

Change-Id: I2719ad7b071ff68ccc1072972aabca3d0f3168d8
Related-Prod: https://mirantis.jira.com/browse/PROD-20427
diff --git a/aodh/server/single.yml b/aodh/server/single.yml
index eba3fc1..df4be81 100644
--- a/aodh/server/single.yml
+++ b/aodh/server/single.yml
@@ -1,2 +1,8 @@
 classes:
 - service.aodh.server.single
+parameters:
+  _param:
+    openstack_event_alarm_topic: alarm.all
+  aodh:
+    server:
+      event_alarm_topic: ${_param:openstack_event_alarm_topic}