Update chpl_launchcmd.py to use select/place syntax with PBSPro.

#1044 updates the launcher code to support select/place syntax for
PBSPro (and includes reasoning). This is the complementary update
to chpl_launchcmd.py.

Factor the qsub command construction into a method on the abstract
class. Implement a version of that method on the PBSPro subclass
that uses the select/place resource list directives (instead of the mpp*
options, which are deprecated and often disabled in practice).

Get rid of a static class member 'processing_elems_per_node_resource';
it was only used for PBSPro and now it is not used at all.
I considered setting these to None, since they are not used. What is your preference?