support running multiple builds in parallel (devstudio 10.1 stable vs. 10.2 CI)

If I run a build of the rh-eclipse46-devstudio rpm from the stable update site (10.1) and at the same time a build from master is triggered by upstream devstudio.product job, this can happen:

Start: run
INFO: Start(devstudio-1.0-1.20160919.1356.el7.src.rpm)  Config(rh-eclipse46)
ERROR: Exception(devstudio-1.0-1.20160919.1356.el7.src.rpm) Config(rh-eclipse46) 0 minutes 0 seconds
INFO: Results and/or logs in: /var/lib/mock/rh-eclipse46/result
ERROR: Build root is locked by another process.

– http://wonka.mw.lab.eng.bos.redhat.com/jenkins/view/devstudio/job/devstudio.rpm_10.0.neon/12/console

So, need to learn how to configure the mock roots so that they're unique by branch/stream/build and don't overlap.

Probably requires configuring the mock.cfg to use unique values in

https://github.com/jbdevstudio/jbdevstudio-product/blob/master/rpm/build.sh#L270-L292