Fix mirror build jobs.

The job name matched the python27 functions regex, but as a post job,
it has no change/branch object.  Make the function safe for post jobs.

Configure them to also be triggerable by zuul (the timer still works
because all zuul variables are ignored).

Configure them to not run concurrently (there's only one matching
executor so it shouldn't be a problem, but be explicit) since they
share an output location.

Change-Id: I210da46bac1a8fa5ac9a931f07f189b3734b2ab0
Reviewed-on: https://review.openstack.org/25646
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
Add statusbot.

Change-Id: I9e1902040389be51db5e31edb84e2f950d5cb3fd
Reviewed-on: https://review.openstack.org/25630
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins