Add redis config for gitlab-kas

What does this MR do?

• Adds Redis configuration for GitLab KAS, as Redis is becoming a required component.
• KAS supports both a single Redis as well as Sentinel.
• TLS support is not yet available, but will get done soon, see gitlab-org/cluster-integration/gitlab-agent#78 (closed)

See also the corresponding Helm chart MR gitlab-org/charts/gitlab!1773 (merged)

Related issues

Test plan

1. Built a version of an omnibus docker image that includes the new changes to gitlab-kas, ran with

   docker run --rm -it $image bash`

2. In the image, configured gitlab with gitlab-kas enabled:

   echo "gitlab_kas['enable'] = true" > /etc/gitlab/gitlab.rb
   /opt/gitlab/embedded/bin/runsvdir-start &
   gitlab-ctl reconfigure

3. Checked that the gitlab-kas configuration at /var/opt/gitlab/gitlab-kas/gitlab-kas-config.yml was as expected

4. Inspected the gitlab-kas logs for errors

   tail /var/log/gitlab/gitlab-kas/current

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
• Equivalent MR/issue for the GitLab Chart opened