integrate collectd slurm plugin to generate data for #189 visualization (https://github.com/hpc/hpc-collab/issues/189)

[ssenator]

See also #168

now integrated into collectd/collectd but originally: https://github.com/pllopis/collectd/tree/slurm
collectd/collectd#3037 <-- merged into main collectd
https://collectd.org/