Factor qsub command list logic into property.
Factor base qsub command (sans resource list args) into its own method.
Allow verbose output to be enabled through environment variable.
Add pbspro specific _qsub_command implementation using select/place options.

select/place syntax is the modern way of specifying a resource list with
PBSPro. This has not yet been tested when a hostlist is specified.
Update launchcmd to work with PBSPro select/place hostlists.

The syntax is crazy (and is probably/hopefully never used in the real world),
so rely on caller to pass in proper form, as opposed to constructing it based
on the old style inputs.
Drop no mppdepth env var; it is no longer needed.

The logic in the _qsub_command for PBSPro implementation will use the host list
expression as the select expression. This means num_cpus_resource is only used
when it is valid, and can always be 'ncpus'.
Merge branch 'master' of https://github.com/chapel-lang/chapel into pbspro-use-select-place-syntax
Merge pull request #1054 from thomasvandoren/pbspro-use-select-place-syntax

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.

r: @ronawho