Add a 'node_distribution' option to the slurm plugin.

[Paul-Ferrell]

It would be nice to be able to select things like:

• contiguous - Just use Slurm's contiguous option.
• semi-contiguous - Select nodes that are contiguous, but allow gaps.
• random - Select a random set of nodes. It would be nice if this was aware of other random selections in the current job, so that multiple random selections could run at once without risk of collision.
• spaced - Space the nodes at a relatively even interval. Should also be aware of selected nodes for other jobs.