zuul layout validation needs a public conf file

/etc/zuul/zuul.conf is only readeable by jenkins user. Puppet change
https://gerrit.wikimedia.org/r/70621 craft a public configuration file
that does not contains the jenkins API key and that we can read as a
jenkins-slave user.

bug: 50223
Change-Id: I5e305fd7f5b4b870c91be495f60e16138739b4d9