Update CRM_Utils_System_Drupal8::authenticate() to Drupal 8.

This function *always* bootstraps Drupal, irrespective of the loadCMSboostrap flag. This might prove to be an issue but is the only reliable way to authenticate Drupal users.
Update CRM_Utils_System_Drupal8::loadBootStrap() to Drupal 8.
Fix checking the existence of constant.