Add 'datasource: influxdb' to Grafana meta

Change-Id: I9daada1cf87873d602d415642bbc4bd32d8e2d52

galera/meta/grafana.yml

@@ -8,6 +8,7 @@ dashboard:
     format: json
     template: galera/files/grafana_dashboards/mysql_influxdb.json
   main:
+    datasource: influxdb
     row:
       ost-middleware:
         title: Middleware