contint: configuration files renaming

* Rename zuul.conf to zuul-server.conf
* Have Zuul point to it by adjusting the 'zuul' default file with:
  DAEMON_ARGS='-c /etc/zuul/zuul-server.conf'
* Forge a public file named zuul.conf with Jenkins API key stripped.
* Drop public.conf

The reason for this change is that the zuul client attempts to read
/etc/zuul/zuul.conf which is zuul/root only, thus we have to either sudo
as zuul or pass -c /etc/zuul/public.conf which is cumbersome.

Change-Id: I954e557aee2b62d2e40ef779daab7f04d8bdb004