psutil.NUM_CPUS has been deprecated in psutil 2.x
While currently global-requirements restrict usage of psutil to
versions <2.0.0 it can be good to be prepared for upgrade in advance.
This commit makes use of multiprocessing module instead.
Change-Id: I6a4653eb7e5c26c3068a4d766acb4bc219920f75
Closes-Bug: #1598208
(cherry picked from commit 8fb54da2f9)