RFR: 8191768 - Introduce a concurrency factor to be able to scale up or down jtreg concurrency when running Hotspot tests

[Christian Tornqvist]

Please review this small change that allows a user to scale up or down the 
concurrency for running Hotspot jtreg tests using CONCURRENCY_FACTOR.

Webrev: http://cr.openjdk.java.net/~ctornqvi/webrev/8191768/webrev.00/ 
<http://cr.openjdk.java.net/~ctornqvi/webrev/8191768/webrev.00/> 
Bug: https://bugs.openjdk.java.net/browse/JDK-8191768 
<https://bugs.openjdk.java.net/browse/JDK-8191768> 

Thanks,
Christian