Implement alarms history cleanup via panko-expirer

Added ability to create cron for panko-expirer launch.
By default expirer will be launched on hourly basis.

Change-Id: I7b128f804d715f417950eba32a4f8be9e74afdac
Related-Prod: https://mirantis.jira.com/browse/PROD-21307
diff --git a/README.rst b/README.rst
index 2a9932b..1cceabc 100644
--- a/README.rst
+++ b/README.rst
@@ -48,6 +48,25 @@
           - host: 10.20.0.104
             port: 11211
 
+Setting event cleanup
+
+In order to allow events cleanup from one node of the cluster,
+server:role field should be set to primary and all others to
+secondary to avoid race conditions. On the example below
+expirer is set to run every day at 2:00 AM. By default
+it will be run every hour.
+
+.. code-block:: yaml
+
+    panko:
+      server:
+        role: primary
+        expirer:
+          cron:
+            minute: 0
+            hour: 2
+
+
 Enhanced logging with logging.conf
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