run slurmctld as user

Notice: also trying to run slurmd on execute nodes as user makes no
sense because it breaks sbatch. Furthermore there is another necessary
to run mpi jobs (just tried MpiDefault=none). I don't consider running
slurmd as root a good idea, but there seems to be no other choice at the
moment.

roles/slurm/defaults/main.yml

@@ -1,4 +1,5 @@
 container_privileged: False
+slurm_user: slurm
 slurm_log_path_ctld: /var/log/slurm/slurmctld.log
 slurm_log_path_d: /var/log/slurm/slurmd.log
 slurm_log_path_sched: /var/log/slurm/slurmsched.log