Add parameter to Nginx proxy for Grafana port

The port number has changed for StackLight v2.

Change-Id: I96bcda8485b5e8e5008769b89acc90f8836b9e31
diff --git a/nginx/server/proxy/stacklight/grafana.yml b/nginx/server/proxy/stacklight/grafana.yml
index a1c7c49..24219ee 100644
--- a/nginx/server/proxy/stacklight/grafana.yml
+++ b/nginx/server/proxy/stacklight/grafana.yml
@@ -1,4 +1,6 @@
 parameters:
+  _param:
+    nginx_proxy_grafana_port: 3000
   nginx:
     server:
       enabled: true
@@ -9,7 +11,7 @@
           name: grafana
           proxy:
             host: ${_param:stacklight_monitor_address}
-            port: 3000
+            port: ${_param:nginx_proxy_grafana_port}
             protocol: http
           host:
             name: ${_param:cluster_public_host}