services: certbot: Get certbot to run non-interactively.

* doc/guix.texi (Certificate Services): Add email field and link to the ACME
Subscriber Agreement.
* gnu/services/certbot.scm (<certbot-configuration>, certbot-command,
certbot-activation, certbot-nginx-server-configurations): Add email field.
(certbot-command): Add '-n' and '--agree-tos' options.
(certbot-service-type): Remove default-value.