Fix Slurm config in prod2312 branch

[sjpb]

• Using Enable use of custom Slurm builds ansible-role-openhpc#163 means openhpc_slurmd_spool_dir has to be specified instead of openhpc_config_extra.SlurmdSpoolDir, so that the role can actually create the spool dir.
• The openhpc_config_extra dict is defined both environments/nrel/inventory/group_vars/openhpc/overrides and in environments/{prod,vtest}/inventory/group_vars/all/openhpc-generic-slurm.yml, which makes it unclear which one is actually getting applied.
• environments/nrel/inventory/group_vars/openhpc/overrides.yml defines openhpc_config_extra.StateSaveLocation - this should be defined using openhpc_state_save_location. NB the set location /var/spool/slurm/slurmctld is not on persistent storage, not addressed here.
• Some general tidying of openhpc_* vars so that these are only contained in 3x files: environments/{nrel,vtest,prod}/group_vars/openhpc/overrides.yml

Not addressed by this PR: openhpc_packages_extra_nrel (used by) openhpc_packages_extra won't be applied here, when using generic slurm, openhpc_generic_packages is used instead. It also contains a lot of openhpc-specific packages.