Use the availability_of_members policy

Use the availability_of_members policy for the elasticsearch_service
alarm cluster definition.
diff --git a/elasticsearch/meta/heka.yml b/elasticsearch/meta/heka.yml
index 11e6c04..a4fae02 100644
--- a/elasticsearch/meta/heka.yml
+++ b/elasticsearch/meta/heka.yml
@@ -60,7 +60,7 @@
 aggregator:
   alarm_cluster:
     elasticsearch_service:
-      policy: majority_of_members
+      policy: availability_of_members
       alerting: enabled
       group_by: hostname
       match: