Allow Slurm federation of multiple Magic Castle clusters

[cmd-ntrf]

To enable Slurm Federation, all clusters must be connected to the same slurmdbd.

Currently, the AccountingStorageHost is not configurable and defaults to the hostname of the instances that includes profile::slurm::accounting.

The clusters also need to share the same munge key, but this can be configured with slurm::base::munge_key.

[mboisson]

Neat idea, but for this to be useful, don't you also need the same underlying storage ?