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.