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 field should only be displayed when user selects the BY_NUMBER option, not sure yet how to do that.