Introduce a concurrency factor to be able to scale up or down jtreg concurrency when running Hotspot tests

The Hotspot test makefile uses numberOfCores / 2 when setting the jtreg concurrency, sometimes there's a wish to scale this up or down based on type of build (product, debug, slowdebug) or flags sent to the test (-Xcomp). A concurrency factor allows a user to scale this up or down.