letsencrypt: Mark module as non-essential
There is an on-going discussion regarding whether the certbot (letsencrypt) package should be in Debian stretch, due to instability of the API: https://lists.debian.org/debian-devel/2016/11/msg00784.html
With this change, the certbot package won't be a dependency for plinth, and won't be installed automatically. Users can still visit the letsencrypt page to setup the module, which will install the certbot package (assuming the package is available).