Support automatic GRES configuration for NVIDIA GPUs

[sjpb]

1. For nodes with non-MIG/vGPU NVIDIA GPUs and an image built including the cuda role, GRES can now be configured by setting:

   # environments/site/inventory/group_vars/all/openhpc.yml:
   openhpc_gres_autodetect: nvml

   Note that:

   • Setting GresTypes in openhpc_config_extra is no longer ever required.
   • For nvml autodectection (only), the conf option in gres entries in openhpc_nodegroups is no longer required.

2. Enables nvml autodetection for the .caas environment, so Azimuth Slurm clusters only need an appropriate image built to autoconfigure NVIDIA GPUs.

For full details see stackhpc/ansible-role-openhpc#202.

[sjpb]

CI failed b/c this needs to be rebased on top of #818.