Powering off unused nodes with SLURM

Having nodes turned on without jobs is a waste of energy (bad for the environment) and reduces the lifespan of the hardware, which is already close to death.

The nodes owl1 and owl2 shoud be kept powered off until a job script is launched. Then the slurm daemon can turn on the nodes, run the job, wait for a grace period without more jobs and then turn them back off.