added grafana-prometheus address variable on system level

to avoid incorrect parameter-resolving

Change-Id: I1cf041d81562ae554595c325d0b0f66668a087bd
diff --git a/grafana/client/datasource/prometheus.yml b/grafana/client/datasource/prometheus.yml
index c4835ef..681b9df 100644
--- a/grafana/client/datasource/prometheus.yml
+++ b/grafana/client/datasource/prometheus.yml
@@ -2,6 +2,7 @@
   _param:
     grafana_prometheus_is_default: true
     grafana_prometheus_ds_name: prometheus
+    grafana_prometheus_address: ${_param:stacklight_monitor_address}
   grafana:
     client:
       datasource: