[FIXED JENKINS-11089] Allow control of fallback jdk usage

Hi there!

I ask you to merge the functionality added in the branch to the jenkins master. Please see a complete problem and feature description here https://issues.jenkins-ci.org/browse/JENKINS-11089.

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