Add example database config for main and ci

What does this MR do?

This MR will add main and ci sections to values.yml. This way, we make it explicit that we use two configurations for the database.

I also added an explanation that global.psql values are used as defaults for main and ci. The linked issue was suggesting to move all settings to both main and ci. But currently, we also need global.psql configuration.

Example:

helm template . \
  --set certmanager-issuer.email=test@example.com \
  --set registry.database.enabled=true \
  --show-only charts/gitlab/charts/webservice/templates/configmap.yml | yq '.data."database.yml.erb"'

The resulting database.yml file will be the same as we now have on master branch:

production:
  main:
    adapter: postgresql
    encoding: unicode
    database: gitlabhq_production
    username: gitlab
    password: <%= File.read('/etc/gitlab/postgres/psql-password-main').strip.to_json %>
    host: "release-name-postgresql.default.svc"
    port: 5432
    connect_timeout:
    keepalives:
    keepalives_idle:
    keepalives_interval:
    keepalives_count:
    tcp_user_timeout:
    application_name:
    prepared_statements: false
    database_tasks: true

  ci:
    adapter: postgresql
    encoding: unicode
    database: gitlabhq_production
    username: gitlab
    password: <%= File.read('/etc/gitlab/postgres/psql-password-ci').strip.to_json %>
    host: "release-name-postgresql.default.svc"
    port: 5432
    connect_timeout:
    keepalives:
    keepalives_idle:
    keepalives_interval:
    keepalives_count:
    tcp_user_timeout:
    application_name:
    prepared_statements: false
    database_tasks: false

Related issues

#4664 (closed)

Author 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
• When ready for review, follow the instructions in the "Reviewer Roulette" section of the Danger Bot MR comment, as per the Distribution experimental MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added/updated
  • The generated templates are not changed due to this MR.
• Integration tests added to GitLab QA
  • The generated templates are not changed due to this MR.
• Equivalent MR/issue for omnibus-gitlab opened
  • gitlab-org/omnibus-gitlab#8317 (closed)
• Equivalent MR/issue for Gitlab Operator project opened (see Operator documentation on impact of Charts changes)
  • No impact expected: the generated templates are not changed due to this MR.
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.
  • No new values

Closes #4664 (closed)