plugins/slurm: Add scripts for job control

- `plugins/slurm/scord_prolog.sh`: Prolog script. Starts `scord-ctl`
   server and sets up environment.
- `plugins/slurm/scord_epilog.sh`: Epilog script. Stops `scord-ctl`
   server and cleans up environment.
- `plugins/slurm/scord_common.sh`: Common code to prolog and epilog.