need to update psutil usage for psutil 3.0+ API

[mperrin]

Anand sent an error report of the following:

     if recommendation > psutil.NUM_CPUS: recommendation = psutil.NUM_CPUS
E       AttributeError: 'module' object has no attribute 'NUM_CPUS'

Turns out the psutil.NUM_CPUS syntax was deprecated and was removed in psutil ≥ 3.0. See http://grodola.blogspot.com/2015/06/psutil-30.html

We need to update poppy to use psutil.cpu_count() instead. Luckily this syntax is sufficiently back compatible to work for anyone who still has psutil 2 installed.