We are using ISPConfig 3 to manage our VPS. We have created a site with a shell user assigned to that site. There is an application installed on the VPS that is called drush (shell application for managing drupal sites). The latest version (7-dev) of drush requires installation through the composer. It is what we have done for the root user following this tutorial. Drush is available and can be used by the root user as well as by any user that was created through the ssh (not ISPConfig's panel) on our droplet. Question: how can we make drush available for shell users that were created by ISPConfig's user interface???