Add support in omnibus to auto-generate ci connection (behind a flag)

What does this MR do?

This MR adds documentation and sample configuration for optional multiple database support.

How to test:

Change the line # gitlab_rails['databases']['ci']['enable'] = false into gitlab_rails['databases']['ci']['enable'] = true and run sudo gitlab-ctl reconfigure

This will change /opt/gitlab/embedded/service/gitlab-rails/config/database.yml to include a section for ci that is a copy of main

Example:

# This file is managed by gitlab-ctl. Manual changes will be
# erased! To change the contents below, edit /etc/gitlab/gitlab.rb
# and run `sudo gitlab-ctl reconfigure`.

production:
  main:
    adapter: postgresql
    encoding: unicode
    collation:
    database: gitlabhq_production
    username: "gitlab"
    password:
    host: "/var/opt/gitlab/postgresql"
    port: 5432
    socket:
    sslmode:
    sslcompression: 0
    sslrootcert:
    sslca:
    load_balancing: {"hosts":[]}
    prepared_statements: false
    database_tasks: true
    statement_limit: 1000
    connect_timeout:
    keepalives:
    keepalives_idle:
    keepalives_interval:
    keepalives_count:
    tcp_user_timeout:
    application_name:
    variables:
      statement_timeout:
  ci:
    adapter: postgresql
    encoding: unicode
    collation:
    database: gitlabhq_production
    username: "gitlab"
    password:
    host: "/var/opt/gitlab/postgresql"
    port: 5432
    socket:
    sslmode:
    sslcompression: 0
    sslrootcert:
    sslca:
    load_balancing: {"hosts":[]}
    prepared_statements: false
    database_tasks: false
    statement_limit: 1000
    connect_timeout:
    keepalives:
    keepalives_idle:
    keepalives_interval:
    keepalives_count:
    tcp_user_timeout:
    application_name:
    variables:
      statement_timeout:

Related issues

Closes #7386 (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 (covered by spec/chef/cookbooks/gitlab/recipes/gitlab-rails/database_yml_spec.rb)
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened