Support for selecting Grid Engine parallel environments

[willfurnass]

The req_nslots trait could be more useful if the parallel environment can be selected. Think something like the following could be added to GridengineSpawner:

req_par_env = Unicode('', 
    help="Select Grid Engine Parallel Environment"
    ).tag(config=True) 

[mbmilligan]

As with the req_cluster and req_partition configurables recently added to SlurmSpawner, we'll accept PRs adding subst variables reflecting terminology peculiar to specific resource managers.

[rkdarst]

If you tell me what the relevant options are, I can add it (at the time time I upgrade to jinja2 syntax for that script).