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/ceilometer/server/backend/default.yml b/ceilometer/server/backend/default.yml
index 542314c..071e4a1 100644
--- a/ceilometer/server/backend/default.yml
+++ b/ceilometer/server/backend/default.yml
@@ -2,6 +2,8 @@
 - service.ceilometer.server.publisher.gnocchi
 - service.ceilometer.server.publisher.panko
 parameters:
+  _param:
+    openstack_event_alarm_topic: alarm.all
   # gnocchi and panko are default backends for Ceilometer since Pike
   # they are incompatible with any legacy database publisher backends
   ceilometer:
@@ -12,3 +14,7 @@
       publisher:
         default:
           enabled: false
+        event_alarm_notifier:
+          enabled: true
+          url: "notifier://?topic=${_param:openstack_event_alarm_topic}"
+          publish_event: true