Certbot role needs to handle first-time registration

[jamezpolley]

In handling #150 it was discovered that the current certbot role does not handle the case where it's never been used with an ACME server before. In that case, it must accept the server's TOS and provide a contact email address in order to create an account before it can register certificates.

A simplistic fix for this was used in b69e10c - simply hard-coding the relevant flags on the command-line.

A better fix for this might be to check for the existence of /etc/letsencrypt/accounts/{{ SERVERPATH }}/. If that's not found, use a supplied email address to register a new account prior to the commands which generate the certificate.