Problem with file loading in a multi-domain configuration (Drupal)

To reproduce :

• have a drupal multi-domain installation
• consult CiviCRM on a domain different than the main one
• configure the CKEditor to fit your need
• create a new mailing -> CKEditor doesn't take the change into account

When i look at the code, we can see that default subdirectory is used :

var CRM = { ... CKEditorCustomConfig":{"default":"https:\/\/civicrm.foundationfuturegenerations.org\/sites\/default\/files\/civicrm\/persist\/crm-ckeditor-default.js"}

The problem is in CRM/Utils/System/Base.php where siteName gives 'default'

@@ -578,6 +578,7 @@ abstract class CRM_Utils_System_Base {
     }
     elseif ($this->is_drupal) {
       $siteName = $config->userSystem->parseDrupalSiteName($civicrm_root);
       if ($siteName) {
         $filesURL = $baseURL . "sites/$siteName/files/civicrm/";
       }

We should instead use the files sites.php to see what is the main domain for the current domain.

See also CRM-17521 for a fix related to a similar issue in 4.6.