Deploy gitaly-blackbox in production

Part of #1737 (closed). Now that !1860 (merged) is merged, we have a new prometheus exporter gitaly-blackbox. The next thing we want is to have it in production.

• write automation (a chef cookbook?) to deploy and configure gitaly-blackbox
• provision or allocate infrastructure to host it
• make sure we run only 1 instance of gitaly-blackbox; it makes expensive HTTP requests
• add to prometheus as a scrape target
• pick some initial repos we want to monitor
• create grafana dashboard