Multiprocessing with n_jobs given by multiprocessing.cpu_count() is not optimal for
systems where not all CPU can be used (in particular, Docker, CI etc).

As suggested in the multiprocessing docs
(http://docs.python.org/3/library/multiprocessing.html#multiprocessing.cpu_count)
using len(os.sched_getaffinity(0)) might be better.

even if the machine have mores cores available

Patch taken from YBD: https://gitlab.com/baserock/ybd/blob/master/ybd/app.py#L227

buildstream/_project.py: Restrict max-jobs

See merge request !620