buildrequest collapse

 - TODO:
   - update master/docs/manual/customization.rst

First iteration of the project.
I plan to update customization.rst.
@djmitche 

Assuming the following scenario:

buildset A:
 - sourcestamp: ssa
 - buildrequest: a1 => SUCCESS
 - buildrequest: a2 => SKIPPED (a1 is collapsed)

buildset B:
 - sourcestamp: ssb
 - buildrequest: b1 => FAILURE
 - buildrequest: b2 => FAILURE

With the current implementation: 
 - buildsetA is complete with status=SUCCESS
 - buildsetB is complete with status=FAILURE.

What you suggest (if I understand correctly), is to complete buildsetA only when b2 is completed.
In this case, what will be the status of the buildset A: Failure ?

It seems weird to me to complete BuildsetA with the result of a sourcestamp which is not related to the buildset.