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

Proposal

1. Add ability to copy ci connection from main connection for charts install method. The ci connection will connect to the same database as the main connection
2. It will be behind a flag (that is suitable for charts install method). The flag will default to false. Customers can flip this to true if they wish to opt-in earlier than %16.0.

The intention is that this flag will flip to true in %16.0

The expectation is that the config/database.yml will then look something like this, if the flag is true:

production:
  main:
    database: gitlabhq_production
    database_tasks: true
  ci:
    database: gitlabhq_production
    database_tasks: false

NOTE: already implemented ! see #4112 (comment 1251145705)