Fix launchcmd script to work with PBSPro 

For qsub, PBSPro requires that you reserve the number of nodes, as well as the
number of cpus per node. This adds that logic to the launchcmd script for
PBSPro (it's not needed for MOAB/TORQUE.)

This behavior matches what we do in our pbs-aprun launcher, and the method for
determining the number of cores to reserve is based off of the code in the
launcher as well.
Since this is a instance attribute (property), it seems more clear to use a noun like `num_cpus` instead of a verb and a noun.