Add 'datasource: influxdb' to Grafana meta

Change-Id: I3f467f10f7c649897f4a0f706cb0f0d9ed7c36a5
diff --git a/rabbitmq/meta/grafana.yml b/rabbitmq/meta/grafana.yml
index 690f32d..10976c1 100644
--- a/rabbitmq/meta/grafana.yml
+++ b/rabbitmq/meta/grafana.yml
@@ -8,6 +8,7 @@
     format: json
     template: rabbitmq/files/grafana_dashboards/rabbitmq_prometheus.json
   main:
+    datasource: influxdb
     row:
       ost-middleware:
         title: Middleware