Implement post-enable hook for 'hosting_*' modules

In #2098389: subdirs kicks in even when subdirs module is disabled., we began writing a /var/aegir/.drush/drushrc.php that contains a 'hosting_features' options array listing the Hosting Features enabled in the front-end. This is updated every time the @hostmaster site in verified and saving the Hosting Features page triggers this verify.

However, Hosting Features enabled using 'drush en' won't be registered until the next @hostmaster verify. I think we need to implement a post-enable hook for 'hosting_*' modules to trigger the verify.