Reduce user interaction in verdi quicksetup for pre-existing user/database

[ltalirz]

Is your feature request related to a problem? Please describe

When verdi quicksetup detects that a database user or database already exists, it prompts the user whether to use those and, if not, to pick a new name.

Novice users likely do not know what exactly this choice entails; furthermore, choosing to reuse the database user may actually not be possible if the correct password for this user is not known in the config.json.

Furthermore, if the user decides not to reuse the db user/db, there is no point in letting them choose the new name. It is much easier to autogenerate it (if the user really wants to choose the name, they can pass it in via the --db-username/--db-name options to verdi quicksetup).

Describe the solution you'd like

verdi quicksetup should ideally behave as follows:

• if a database user with the suggested name already exists, check whether you can log in as this user. if so, use it. if not, create a new user with an autogenerated name (printing a message to the cli about this)
• if a database with the suggested name already exists, ask the user whether to reuse it. if they answer no, create a new db with an autogenerated name.

Additional context

[ltalirz]

On second thought, let's not reuse a database if it exists.
Quicksetup should only be used to create new databases - for existing ones, one can use verdi setup.