database: allows hostname parameter be None

It was 'localhost' and empty space before, which was inconsistent
and unclear to handle other engines like postgresql which allows
to connect via socket directly.

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
Update the data model to make it consistent with the code
Clean up of data model scripts

Remove duplicated script and renamed the general one to make
it explicit that refers to MySQL