use certbot-auto --non-interactive flag

[ebarault]

Hi, Many thanks for your work.

While using the cert-renewal-haproxy.sh with cron as root, I experienced an issue with dependancies the certbot-auto bootstraper was trying to update, using interactive mode.

Cerbot-auto bootstrap was calling apt-get install without the default yes flag, thus the script was aborting, as apt-get was not receiving y to continue.

Using the --non-interactive flag in the issuecert shortcut function allows to force the bootstraper using default yes flag (-y) with apt-get.