RFC: Support for DRMAA backend

[Ricardo Wurmus <rekado@elephly.net>]

Hello there,

I pushed a new branch “wip-drmaa”, which lets GWL use an HPC 
scheduler via DRMAA.  Set the GWL_USE_DRMAA variable and use the 
default execution engine.  (In the future this variable will 
disappear once we have agreed on a nicer way to select DRMAA 
execution.)

I only tested this with a local Slurm cluster on my laptop, but it 
seems to work fine.  I’d be happy to hear test reports!

Before this can make its way into the main branch we have to 
figure out if the concept of an execution engine still makes sense 
the way it is implemented.

Execution engines currently are only used to build custom scripts 
(or script wrappers) — but using DRMAA does not require any new 
kind of script.  The old Grid Engine backend builds the usual 
workflow scripts and a set of job scripts that call those scripts 
after passing Grid Engine settings.  For DRMAA we only need 
run-time support.

I wonder if we should scrap execution engines as they currently 
exist; we would only ever generate the same kind of workflow 
scripts and move all execution-related work (like submitting jobs 
to a scheduler) to, well, execution time.  This also means that 
the workflow scripts would not need to be modified when picking a 
different execution backend, which I think is good.

Comments?

-- 
Ricardo