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.
Hm, perhaps a switch to 'numcpus'? I understand your point, however I figured it would be an indication of the maximum amount of CPUs available for use on the slave.