Add system.prometheus.*.container classes

Change-Id: I19ecb0485770f973ef25a4daa38198b293c5bf6c
diff --git a/prometheus/pushgateway/container.yml b/prometheus/pushgateway/container.yml
new file mode 100644
index 0000000..d0e986e
--- /dev/null
+++ b/prometheus/pushgateway/container.yml
@@ -0,0 +1,2 @@
+classes:
+- service.prometheus.pushgateway.container