Restore slurm tracker?

There exists code for using the SLURM scheduler as a tracker for distributed training, but it was removed as an option from `submit.py` some time ago.

Lately I've been training XGBoost using an MPI cluster and while I haven't been able to get the `mpi` tracker to work, re-instating the SLURM tracker seems to work, after I made some changes to the command being called.

So would the community consider adding back SLURM as an option or is it supposed to be superseded by the `mpi` tracker now? In that case has anyone gotten the MPI tracker to train XGBoost recently?