Ensure Zuul points to existing jobs

Fetch the list of defined jobs using jenkins-jobs-list.py which I have
added with https://gerrit.wikimedia.org/r/156540

Zuul layout validation has an option parameter which let you pass a list
of defined jobs and bails out whenever we defined a trigger to a non
existing job.

Change-Id: Ifbf559213cc24d7f95fcc9ee25d022779fa2c1c1