Add slurm support

[dhdaines]

While various PBS-like schedulers still live on, it seems that most people use Slurm these days, and there's even a relatively straightforward tutorial on setting it up on Google Compute Engine ... which looks significantly more helpful than the "documentation" for CycleCloud on Azure.

Elasticloud still exists too, it seems to perhaps support slurm better than others.

So we should support slurm, it should be pretty easy given the small subset of scheduler functionality we need.