The intent of the periodic jobs is to run with latest master. If
they get enqueued, then other patches land, they'll still run with
the value of the zuul ref from when they were enqueued. That's not
what we want for prod, as it can lead to running old versions of
config.
We don't usually like doing this, but in this case, rather than
making us remember to add a flag every time a prod job gets added
to a periodic pipeline, how's about we just calculate it.
Change-Id: Ib999731fe132b1e9f197e51d74066fa75cb6c69b
32978fe729