Add Nagios class to the metric_collector metadata

This change adds a Nagios output class to the metric_collector service
metadata to be on par with Sensu.

Change-Id: Ib094961cf395c451ad6517ab73f928d3ef6537fc
diff --git a/metadata/service/aggregator/output/sensu.yml b/metadata/service/aggregator/output/sensu.yml
index 394acb6..37b2cf8 100644
--- a/metadata/service/aggregator/output/sensu.yml
+++ b/metadata/service/aggregator/output/sensu.yml
@@ -1,5 +1,7 @@
 parameters:
+  _param:
+    sensu_source_dimension_key: nagios_host
   heka:
     aggregator:
-      sensu_source_dimension_key: nagios_host
+      sensu_source_dimension_key: ${_param:sensu_source_dimension_key}
       sensu_host: 127.0.0.1