Expose build, launcher, listener to groovy scripts run on the master node

This is somewhat of a fishing pull request.  I can't tell who (if anyone) has an interest in maintaining or enhancing scriptler.

I'm looking to do some transforms on build axis values before passing them to later build steps. Since groovy scripts don't seem to be directly usable in many build step parameter fields, I'm thinking a groovy script build step might be a workable solution, and scriptler seems like a nice solution for managing the scripts.