Add redis_yml_override setting

What does this MR do?

Add redis_yml_override which renders a new gitlab-rails config file: config/redis.yml. This file acts as an override for all other resque.yml style files. By default it is empty.

The intended use is to allow the administrator to inject arbitrary values into redis.yml. This will make it easier to define and maintain complex Redis configuration on GitLab.com. Also see gitlab-com/gl-infra/scalability#2063 (closed).

Companion Helm MR: gitlab-org/charts/gitlab!2907 (merged).

Related issues

• gitlab-com/gl-infra/scalability#2063 (closed)
• gitlab-com/gl-infra/scalability#2120 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA see !6609 (comment 1247228299)
• Equivalent MR/issue for the GitLab Chart opened