Make resource constraints first class citizens

[jakirkham]

Various schedulers have different ways of specifying resource limits for jobs (e.g. memory, CPU, etc.). It would be nice to explore making these first class citizens in dask-drmaa. As DRMAA doesn't really have a way to specify this itself, it would amount to detecting the DRMAA implementation used and passing along the right flags to the nativeSpecification parameter of the JobTemplate. That said, we have already had some success with this strategy w.r.t. log files as demonstrated by PR ( #56 ). So should be pretty reasonable to extend the strategy to this case as well.

ref: https://www.ibm.com/support/knowledgecenter/en/SSWRJV_10.1.0/lsf_admin/resource_usage_limits_supported.html
ref: https://slurm.schedmd.com/sbatch.html
ref: http://gridscheduler.sourceforge.net/htmlman/htmlman5/queue_conf.html
ref: http://apps.man.poznan.pl/trac/slurm-drmaa#Nativespecification