Support for DRUPAL_SETTINGS as an override for Drupal configuration

There are many different Drupal development and hosting platforms that would like to inject their settings into a Drupal installation without formally writing it to the settings.php.

This pull request allows this through a $_SERVER['DRUPAL_SETTINGS'] value that can be injected in a variety of ways. This is the method we have used at Pantheon to inject our settings for years, but would love for it to be more cleanly supported in Backdrop.