Update reference repos rebooting jenkins

[pshipton]

https://openj9-jenkins.osuosl.org/view/Infrastructure/job/Update-Reference-Repos is causing jenkins to reboot. It was running on Sunday so I moved it to Monday and weekend builds didn't have any issue. The build on Monday rebooted. https://openj9-jenkins.osuosl.org/view/Infrastructure/job/Update-Reference-Repos/227

One thing we can do is break it up to run on subsets of machines, probably by platform as we do for the cleaning jobs.

I also noticed the job is configured to load from https://github.com/adambrousseau/openj9.git serial_update_ref_job branch.

@AdamBrousseau