Drush::dl and drush::en run every time puppet does

(per comment #2 by @ergonlogic in #2072873: Aegir::Extras doesn't work due to old variables:)

...unless we specify the site path, there's no way to tell puppet which directory was created by downloading the module (thus stopping it from trying to download it again.)

We could add an 'unless' clause to drush::dl that does something like:

drush ${site_alias} pm-list | grep ${name}

And similarly for drush::en:

drush ${site_alias} pm-list --status=enabled | grep ${name}