Only allow mail from jenkins from zuul

If someone copies the production Zuul configuration, they may end
up sending emails as jenkins@o.o.  Configure our hosts to only
accept email with an envelope sender or From header of jenkins@.o.o
if the connecting host is zuul.openstack.org (or localhost, to handle
the case of exim running on zuul.o.o itself).

Change-Id: Ic92bb39659689347fea1744ee8c1b47ef21858b0
Merge "Only allow mail from jenkins from zuul"