Adjust Grafana dashboard to represent the one used for the fleet (#1139)

This means adding the instance and container variables which can
be used to select the metrics of the specific fleet node.

fluffy/README.md

@@ -139,6 +139,15 @@ The steps on how to set up such system is explained in [this guide](https://nimb
 
 A Fluffy specific dashboard can be found [here](./grafana/fluffy_grafana_dashboard.json).
 
+This is the dashboard used for our Fluffy testnet fleet.
+In order to use it locally, you will have to remove the
+`{job="nimbus-fluffy-metrics"}` part from the `instance` and `container`
+variables queries in the dashboard settings. Or they can also be changed to a
+constant value.
+
+The other option would be to remove those variables and remove their usage in
+each panel query.
+
 ## For Developers
 
 When working on this repository, you can run the `env.sh` script to run a