Enable pushgateway persistent storage

Change-Id: Ic850cadd55b842c784d9271dc62cc46b389604a1
Closes-bug: PROD-21073
diff --git a/prometheus/pushgateway/container.yml b/prometheus/pushgateway/container.yml
index d0e986e..e4e9cd7 100644
--- a/prometheus/pushgateway/container.yml
+++ b/prometheus/pushgateway/container.yml
@@ -1,2 +1,3 @@
 classes:
 - service.prometheus.pushgateway.container
+- system.prometheus.pushgateway