Launching dask-mpi clusters

Is this supported? We have the ability (through Slurm) to `srun ... dask-mpi...` which will launch the cluster on a queue set aside for interactive work. From the command line it's a single command. Other facilities may not have this capability (it may require a batch script).

We're willing to take a look into how to make this work if it's needed. cc @shreddd

Start at the dask-mpi command-line entry point and inspect how launching through Slurm's `srun` is handled. Determine whether interactive queue launches are supported; done means a documented, working path for `srun ... dask-mpi`, or a clear explanation of batch-script requirements and unsupported facilities.

Written by the indexing model from the issue text.