Resolve "Mailroom: configured Redis server must be the global.redis.queues instance, if present"

Redis: make sentinel template self-determinate

Make the `gitlab.redis.sentinels` template self-determinate on content.
This allows it to be called with or without settings being present.

Closes #2086 (closed)

Relates to #2097 (closed)

Mailroom: template global.redis.queues is present

Mailroom is effectively a job-spawner for Sidekiq. As such, it should
follow the `global.redis.queues` if present, so that those queues are
properly populated for Sidekiq via the server it speaks to.

Closes #2097 (closed)

Spec: check redis sentinels and mailroom urls

- Add redis sentinel configuration when global.redis.sentinels missing
- Add checks for redis configuration of `mail_room.yml`

cc @skarbek @jarv I also happened to solve #2086 (closed) while I was in here fixing this 👍