Add CSCS cluster name to elastic metadata

[msimberg]

When sending data to elastic from CI, we currently only send the slurm partition name and hostname, but not the cluster name. We should add that so that we can distinguish between the same partition names on different clusters.

Edit: We do send SLURM_CLUSTER_NAME, which is set on eiger. However, it seems like on clariden this is not set. Find a more reliable environment variable, or figure out if it should be set on clariden as well.