BuildslaveChoiceParameter and EnforceChosenSlave

This is a patch for a new ForceScheduler parameter to let the user select the buildslave to use. 

There are two parts:
 * the parameter
 * the functor to enforce it

So some questions about this patch:
 * should EnforceChosenSlave be more built in? Or opt-in as I did it here?
 * if opt-in, where should it be?

For this patch, I made it opt-in and next to Builder, but I'm not sure that's the right thing.