Add stacklight_environment parameter

This allows Nagios to support several environments.

https://github.com/Mirantis/mk-lab-salt-model/pull/197
diff --git a/heka/metric_collector/single.yml b/heka/metric_collector/single.yml
index 960d271..443a4e9 100644
--- a/heka/metric_collector/single.yml
+++ b/heka/metric_collector/single.yml
@@ -1,5 +1,6 @@
 classes:
 - service.heka.metric_collector.single
+- system.heka.server
 parameters:
   heka:
     metric_collector: