Discussion about automatic database selection

[luixxiul]

This is a tracking issue for the discussion on #1052

Currently the playbook is configured to use SQLite automatically, if a service supports both SQLite and Postgres, and Postgres is not enabled on the playbook. The issue here is that once Postgres is enabled, every service is switched automatically to use it, which can make users wondering where the data is gone.

One of the solutions to the issue is that making selection of databases full opt-in: the playbook pre-selecting none of them and letting users of the playbook choose a database explicitly if multiple ones are available for each service. This is one of the conservative approaches and mostly should work, but its caveat is that it will be annoying to specify a database engine if you have enabled a lot of services which support multiple engines.