strip drupax-X.X in tarball when extracting

The goal as I understand it is that `$docroot` be effectively the top directory for the drupal installation (including the `sites` directory). In order to achieve this, we need to strip the top-level directory in the archive (which is named `drupal-X.X`) and then the untarring creates a file inside this directory (module users are responsible for creating `$docroot` themselves).