Allow specifying additional databases to Rails application via database.yml

What does this MR do?

Allow users to specify additional databases to be used with the rails application, via database.yml

1. Users can specify additional databases and relevant settings like the following

   gitlab_rails['databases']['main']['db_database'] = 'gitlab_foobar'
   gitlab_rails['databases']['ci']['enable'] = true
   gitlab_rails['databases']['ci']['db_database'] = 'gitlabhq_production_ci'
   gitlab_rails['databases']['ci']['db_connect_timeout'] = 60 # If not provided, will be copied from main database value

2. Any additional database not supported by GitLab (for now, ci is the only supported one) will be ignored and a warning will be shown.

Related issues

Closes #6310 (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
• Equivalent MR/issue for the GitLab Chart opened