Add 'datasource: influxdb' to Grafana meta

Change-Id: I5efd4188476602c45477f8fcea043b7ec11ef2fe
diff --git a/docker/meta/grafana.yml b/docker/meta/grafana.yml
index 010a4ca..074ff06 100644
--- a/docker/meta/grafana.yml
+++ b/docker/meta/grafana.yml
@@ -8,6 +8,7 @@
     format: json
     template: docker/files/grafana_dashboards/docker_influxdb.json
   main:
+    datasource: influxdb
     row:
       docker-data-plane:
         title: Docker