Add 'datasource: influxdb' to Grafana meta

Change-Id: I83e5fb4d5897fd6c4fe93741a8d84e6d8fb97fc6
diff --git a/haproxy/meta/grafana.yml b/haproxy/meta/grafana.yml
index 03339cf..63ab766 100644
--- a/haproxy/meta/grafana.yml
+++ b/haproxy/meta/grafana.yml
@@ -8,6 +8,7 @@
     format: json
     template: haproxy/files/grafana_dashboards/haproxy_influxdb.json
   main:
+    datasource: influxdb
     row:
       ost-middleware:
         title: Middleware