At this moment ( 8.0.0-beta16 ) to switch to file configuration before install Drupal:
Into settings.php
Uncomment
$settings['bootstrap_config_storage'] = array('Drupal\Core\Config\BootstrapConfigStorageFactory', 'getFileStorage');
then add:
$config_directories = array(
CONFIG_ACTIVE_DIRECTORY => 'PATH_OUTSIDE_WEB_ROOT/config/active/',