[JENKINS-18607] Adding an option that uses the exact same strategy from ...

Ok, now this is the correct pull request. I added a checkbox that enables the plugin to randomly assign an *available* display port using a "DisplayAllocator" (I copied the class from the XVnc plugin and renamed).

Here's the issue I addressed:

https://issues.jenkins-ci.org/browse/JENKINS-18607

I also added a global section to configure the range of display numbers that will be picked.