allow shared tables between sites

For our purposes, it is sometimes to share tables between sites. In particular, we share the user table quite regularly. Since this is done in the settings.php, it would require to be a Aegir module of some sort.

I would picture starting with a provision module that would tap into the settings.php hooks to add a $db_prefix variable like this:

  $db_prefix = array('default' => '', 'users' => 'other_db.');

This could be hardcoded in the site's settings to start with, but then we would need a hosting frontend to manipulate that setting. Not sure about the UI here...

The backend should also ensure the user has access to the other_db using the appropriate grant (and also remove those grants when the site is removed).