Trigger periodic jobs with Zuul

This takes care of all the current jobs started by a timer in
Jenkins except the devstack-gate maintenance jobs.  For the time
being, there will continue to be a set of those jobs for each
Jenkins master (as opposed to other periodic jobs which we want
to run only once globally).  In the future we can evolve devstack-
gate to run as a single daemon and remove those jobs.

Change-Id: I1215b7cb51ffcd65b69d1b54e9f8d051a406a57a
Merge "Trigger periodic jobs with Zuul"