Issue 3019.5

This should add the property 'maxcpus' to a buildslaves configuration. If 'maxcpus' is not specified it defaults to 'None' which uses 'multiprocessing.cpu_count()' to receive the max amount of CPUs.
That sentence might give the wrong expectation that one can restrict the amount of core the build will take by setting this to 1 or 2. Whereas it's just meant as an indication of the amount of available CPU cores on the slave ...