Error on running on a host with an existing certificate

[rfleschenberg]

When running against a host that already has a certificate, I run into this error:

[...]
"failed": true, "rc": 1, "start": "2017-02-06 14:39:29.266513", "stderr": "Saving debug log to /var/log/letsencrypt/letsencrypt.log\nCert not yet due for renewal\nAn unexpected error occurred:\nEOFError: EOF when reading a line\nPlease see the logfiles in /var/log/letsencrypt for more details."
[...]

The problem seems to be that certbot prompts for what to do. Passing the -n option seems to fix this. I'll send a PR.