Allow configuration of Slurm node "weights" in the Ansible playbooks

[rhliang]

Slurm allows nodes to be weighted differently to allow jobs to be preferentially assigned to certain nodes, e.g. jobs can be assigned to faster nodes before they get assigned to slower nodes:

https://slurm.schedmd.com/slurm.conf.html#OPT_Weight

Add this to our Ansible playbooks to allow better performance tuning of our clusters.