update scheduler with drmaa config

[cehbrecht]

Overview

This PR updates the scheduler extension to allow configuration of DRMAA (wrapper for slurm job scheduler).

Changes:

• added drmaa_native_specification configuration paramater.
• updated docs for drmaa_native_specification configuration.
• allow unlimited maxprocesses with -1. Same as for parallelprocesses. This is useful when the job queue is managed by a scheduler like slurm.

Related Issue / Discussion

Additional Information

• https://slurm.schedmd.com/documentation.html
• https://github.com/natefoo/slurm-drmaa
• https://github.com/pygridtools/drmaa-python/blob/ed6b926074d730f6fc37505360f65b8b79e8797f/drmaa/session.py#L96

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[coveralls]

Coverage decreased (-0.06%) to 75.344% when pulling a3017b2 on cehbrecht:update-scheduler-with-drmaa-config into 7fd3569 on geopython:pywps-4.2.

[cehbrecht]

@tomkralidis thanks :)