grafana image tweaks
Created by: uwedeportivo
Small cleanups from Stephen's code review of https://github.com/sourcegraph/deploy-sourcegraph-docker/pull/41:
- anonymous user is admin
- env vars pulled into .ini inside image (they can still be overridden with envs)
- delete container.json dashboard
I will spend the rest of the weekend doing dashboard triage and perhaps creating a rpc frontend dashboard (rpc from frontend to backends) using jsonnet. So most likely one more rev after this one on the Grafana image, but want to land this first in case the other stuff won't make it before Monday morning. As usual this PR is accompanied by respective PRs for k8s and deploy-docker.