buildrequest collapse

 - TODO:
   - update master/docs/manual/customization.rst

First iteration of the project.
I plan to update customization.rst.
AFAIU, this routines is called from the master that schedules the buildrequests.

This means that the builder must be configured in the scheduling master, which is not true theorically. Probably identically configured masters in multi-master mode would simplify a lot of things, but this is not the original design.

I cannot think of obvious solution for this problem, and I would be open to just document this restriction and move forward.