Add missing JSON format for dashboard

Change-Id: I31c7072182aeec27b063b9f4639e7879282184d7
diff --git a/heat/meta/grafana.yml b/heat/meta/grafana.yml
index e6bb975..e1cc983 100644
--- a/heat/meta/grafana.yml
+++ b/heat/meta/grafana.yml
@@ -1,3 +1,4 @@
 dashboard:
   heat:
+    format: json
     template: heat/files/grafana_influxdb.json