Install script assumes cluster name should be used for /sched/<clustername> config

When trying to use the install script on "external" nodes, it would be good to be able to specify the cluster name to use rather than imply it from the CC settings. This would help a lot with manual installation of Slurm using this script for things like login visualization nodes that might already have the mounts and everything else present.

I'd propose something like `--clustername` as an argument to the script.