Service discovery for database load balancing

We support service discovery for database load balancing but apparently we're not using it (I looked at a database.yml in production, it seems this is not configured).

Service discovery was implemented with https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5883/diffs by @yorickpeterse .

(I'm not sure yet where this issue is going - but I'm inclined to think that we need service discovery to support flawless failovers.)