Site alias command only needs to bootstrap to drupal root.

The site alias command declared that it needed to bootstrap to DRUSH_BOOTSTRAP_DRUPAL_SITE, but actually DRUSH_BOOTSTRAP_DRUPAL_ROOT is sufficient. When you bootstrap just to the drupal root, then you can still list all local sites with drush -r /path/to/drupal site alias; without this change, you'd need to specify -l too.