Add --dashboard/--no-dashboard flag and enable by default

[jacobtomlinson]

Closes #126

Currently when running dask-mpi the dashboard is disabled unless you set the --dashboard-address flag to some value.

# Old behaviour

# Dashboard disabled (the default)
mpirun -np $SLURM_NTASKS dask-mpi --scheduler-file scheduler.json

# Dashboard enabled
mpirun -np $SLURM_NTASKS dask-mpi --scheduler-file scheduler.json --dashboard-address :8787

In the dask scheduler CLI there is a flag to enable/disable the dashboard and it defaults to enabled. This PR adds the same --dashboard/--no-dashboard flag to the dask-mpi CLI and also sets the default as enabled.

# New behaviour

# Dashboard enabled (the default)
mpirun -np $SLURM_NTASKS dask-mpi --scheduler-file scheduler.json

# Dashboard disabled
mpirun -np $SLURM_NTASKS dask-mpi --scheduler-file scheduler.json --no-dashboard

[kmpaul]

This looks good to me. Thanks, @jacobtomlinson

[kmpaul]

Don't know why ReadTheDocs doesn't install a more recent version of Sphinx than 4.5, but that's why the docs are failing to build. I'm going to merge this and fix the docs error later.

[kmpaul]

Incidentally, the RTD build errors can be fixed with the amendments to the pip section of the docs/environment.yaml conda env file, as mentioned in dask/dask-sphinx-theme#68. I've implemented the local mods in the rank-replacement branch.