Switch to snapd version of certbot

Convert role to use snapd version of letsencrypt (#8)

* Let the role survive check mode

* Replace all old versions of certbot/certbot-auto with snapd version

* Remove obsolete flag

* Update meta

* Remove symlinks; snap doesnt recognize 'certbot-auto' as a valid app name

* Stop breaking mode on this dir; groups still need read/list, and new files should inherit group