Support multiple fabrics

[jabl]

In some situations it would be useful to have multiple fabrics in a cluster (e.g. a long-lived cluster that gets upgraded over time). To support that the ufm<->slurm integration script would need to be able to communicate with several different UFM instances.

In such a case the integration scripts would need to look at e.g. the SLURM_JOB_NODELIST environment variable to determine which UFM instance to connect to.