cwl: Slurm compute backend example
Works well:
$ reana-client status -w roofit-cwl-hpc
NAME RUN_NUMBER CREATED STARTED ENDED STATUS PROGRESS
roofit-cwl-hpc 1 2021-02-25T10:43:44 2021-02-25T10:43:45 2021-02-25T10:53:29 finished 2/2
$ reana-client ls -w roofit-cwl-hpc | grep plot.png
outputs/plot.png 15450 2021-02-25T10:53:24
cwl/docker_outdir/plot.png 15450 2021-02-25T10:53:24