Add task/option to auto-generate cert with Let's Encrypt

[jcalazan]

Also add an option to include a cron job to auto-renew the cert.

Currently thinking of doing something like:

(before Nginx installation)

1. Install certbot
2. Run standalone command, pull fqdn from an env var.
3. In the Nginx template, if option nginx_use_lets_encrypt is true, simply reference the cert and key to the default location created by certbot.
4. Cron job to auto-renew monthly (cert is good for 3 months).

If someone has done this before, would love to hear your solution (or open a pull request :D)!