Add configuration for running Grafana to monitor BuildGrid

This adds a config to run a Grafana instance to monitor BuildGrid.

It borrows the configuration and dashboards that are used by the docker-compose setup.

Grafana is exposed on TCP port 3000, and a Graphite datasource is configured to read from http://statsd:8080 (we might need to change that last URL once we add a Graphite Kubernetes deployment with #4 (closed)).

How to test

> minikube start
> kubectl create -R -f kubernetes/grafana/
> kubectl get deployments # This should list a "grafana" deployment. Wait until it shows "READY 1/1".
> minikube service grafana --url=true

The last command returns a URL. Open that in the browser and it should load Grafana's web interface (no login). There should be two dashboards shown: "BuildGrid Cleanup" and "BuildGrid Scheduler".

(In case of errors, the commands kubectl describe pods, kubectl describe deployment/grafana and kubectl logs deployment/grafana might be useful to determine what is happening.)

Closes #5 (closed)