multipatch job not working

the multipatch job does not seem to be working as expected. Here is a gerrit
where I left the comment "multipatch-build:topic=mri-magnesium-sr2" which
triggered this job but the PATCHES_TO_BUILD parameter is not "topic=mri-magnesium-sr2"
as we'd expect so the job fails.

The workaround is to manually start the job and fill in that parameter, like I did
here

Not everyone can start jenkins jobs manually, so we should fix this if we want to
encourage others to eventually start doing this kind of work.