Unify "give me some number of CPUs I can use for subtasks" logic
Summary: Basically we always want to use as many cores as possible, leaving like 2 or so to be nice to other processes
Reviewers: mmodell, Release-Engineering-Team
Reviewed By: mmodell, Release-Engineering-Team
Subscribers: jenkins
Tags: Release-Engineering-Team
Differential Revision: https://phabricator.wikimedia.org/D828