Make Nagios an optional feature

Now that Sensu is the recommended backend for alerting, it should be
possible to deploy StackLight without Nagios. Unfortunately the current
aggregator and metric_collector classes have a hard dependency on Nagios
and this change fixes it.

Note that user models already using Nagios and willing to stick with it
should now include heka.metric_collector.output.nagios and
heka.aggregator.output.nagios classes wherever needed.
diff --git a/heka/metric_collector/output/nagios.yml b/heka/metric_collector/output/nagios.yml
new file mode 100644
index 0000000..89d981f
--- /dev/null
+++ b/heka/metric_collector/output/nagios.yml
@@ -0,0 +1,2 @@
+classes:
+- service.heka.metric_collector.output.nagios