Proposed use of New JUnit ParallelComputer in Surefire

I agreed with @krosenvold to push the first changes on the use of ParallelComputer in Surefire.

We have to decide whether we should keep using provider 4.8 with new ParallelComputer  or we would rather create a new provider for JUnit 4.12.

Remember that JUnit 4.12 is not yet released.
If you want to see the changes made on JUnit ParallelComputer, see this pull
https://github.com/KentBeck/junit/pull/523