JENKINS-23659 - First pass at allowing tracked build to be specified by a parameter at build time

Hi,

First pass at getting something working to allow tracked build to be specified by a parameter.
Still trying to learn how to do things correctly in jenkins so putting it out here to get some feedback and advice!

many thanks
Geoff
This is the value currently displayed, changed to say Next Tracked build as we will not have actually tracked it until we do a build.