Remove configuration from image

[sjpb]

• Removes etc_slurm named volume as unnecessary
• Mount slurm.conf and slurmdbd.conf in from host
• Note slurmdbd.conf has to be mounted in rw, then the entrypoint script chmods it to slurm inside the container.
• Tested that you can change slurm.conf on the host, then run scontrol reconfigure inside e.g. slurmctld, and changes are propagated to slurmds