Populate Grafana dashboards automatically

Problem Statement

We now have Grafana integrated (#3487 (closed)) with Omnibus, making it very easy to get started monitoring your instance.

However there are no dashboards that are included out of the box, which means users still need to go get them manually.

Proposal

We should automatically provision useful Omnibus dashboards. We should include the Omnibus dashboards from https://gitlab.com/gitlab-org/grafana-dashboards.