Change hardcoded host for sensu/graphite/sphinx in nginx

Change-Id: I939da0cc704174a5e453cc9e02ddbafc73a6a503
diff --git a/nginx/server/proxy/graphite_web.yml b/nginx/server/proxy/graphite_web.yml
index 02035cd..2f3e1e8 100644
--- a/nginx/server/proxy/graphite_web.yml
+++ b/nginx/server/proxy/graphite_web.yml
@@ -8,7 +8,7 @@
           type: nginx_proxy
           name: graphite
           proxy:
-            host: mon
+            host: ${_param:stacklight_monitor_hostname}
             port: 80
             protocol: http
           host: