scontrol: add support for the SLURM_CLUSTERS environment variable

On our frontend host we support multiple clusters (Cray and non-Cray) by
setting the SLURM_CLUSTERS environment variable accordingly.

In order to use scontrol (e.g. for hold/release of a user job) from a
frontend host to control jobs on a remote Cray system, we need support for
the SLURM_CLUSTERS environment variable also in scontrol.