Zuul public.conf is gone

The default /etc/zuul/zuul.conf is made readable with
https://gerrit.wikimedia.org/r/162584 and removes public.conf entirely.

The private server file is made zuul-server.conf which we do not need
for layout validation.

Change-Id: I954e557aee2b62d2e40ef779daab7f04d8bdb004