Add config + docs for new redis-sessions

What does this MR do?

Adds ability to use a separate Redis for session data.

Application support for this is not done yet, so using these values is currently a no-op.

I did not add any tests; was going by what !2187 (merged) did, but happy to take pointers.

Related issues

• gitlab-com/gl-infra/scalability#1311 (closed)
• !2187 (merged)

Test plan

• helm upgrade gitlab . --set global.redis.sessions.host=gitlab-redis-master.default.svc
• shell into webservice pod
• cat /srv/gitlab/config/redis.sessions.yml

File should exist and contain:

production:
  url: redis://:aG3LuirbuhljyG8jt69eZMsBX9VfLX5elZREdO8rWWlLZBr64qna4f5WB6LrHGeR@gitlab-redis-master.default.svc:6379
  
  id:

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

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 -- since Rails support isn't complete, we cannot do this yet.
• Equivalent MR/issue for omnibus-gitlab opened: gitlab-org/omnibus-gitlab!5690 (merged)