I had the same problem today, so I looked in the source code.
In civicrm.install there's a function called _civicrm_get_db_config. It's looking in the $databases array for an element with a key civicrm. So I added this to settings.php:
$databases['civicrm']['default'] = array (
'database' => 'civi_db_name',
'username' => 'mysqluser',