zuul: craft a publicly readeable configuration file

To test out Zuul configuration, we are running it as user jenkins-slave
which does not have access to /etc/zuul/zuul.conf.  This patch creates a
publicly redeable version that has the Jenkins API key stripped out.

bug: 50223
Change-Id: I763994475e80a09449bf7f16b78adc7d18e79e20
Merge "zuul: craft a publicly readeable configuration file" into production