Add watchdog for Heka services

This patch adds a new 'heka.sandbox.watchdog' type of Heka message that
is used by Sensu to check the availability of a service. In the current
implementation the message is issued by a filter for all Heka services
and the check is sent locally to the Sensu client.

Change-Id: I6b3c9d808dcec7d8c15c442390dfbda2032f38ce
diff --git a/metadata/service/aggregator/output/sensu.yml b/metadata/service/aggregator/output/sensu.yml
index bd5e55a..394acb6 100644
--- a/metadata/service/aggregator/output/sensu.yml
+++ b/metadata/service/aggregator/output/sensu.yml
@@ -2,4 +2,4 @@
   heka:
     aggregator:
       sensu_source_dimension_key: nagios_host
-      sensu_host: 127.0.0.1
\ No newline at end of file
+      sensu_host: 127.0.0.1