[JENKINS-11121] Scheduler now checks for requested JDK prior to launching the job on an executor

I ask you to merge these changes into jenkins master. Please see a complete problem and feature description here https://issues.jenkins-ci.org/browse/JENKINS-11121.

Again, the feature was tested exhaustively. Since the enhancement needs a rather complex jenkins setup, unit testing is a huge challenge and is therefore not done (yet). I will try to add some tests as soon as I have some loose time here at work.

Thanks!
Florian