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 think the way _qsub_command is implemented, I can drop the env var... Now, if a hostlist is specified, that becomes the entire select expression for pbspro, so we don't need to set num_cpus_resource to None. It will be ignored when the hostlist env var is set.