Support additional nodegroups

[sjpb]

Important

I'm not sure the name additional_nodegroups is the best? Others are compute and login, although in a way it would be better if they contained _nodegroups too.

• Adds opentofu variable additional_nodegroups to support defining non-Slurm nodes in the cluster. E.g.:

  additional_nodegroups = {
      squid = {
          nodes = ["squid-0"]
          flavor = var.other_node_flavor
      }
  }

  Nodes are automatically added to an inventory group of the same name as the node group.
  Slurm-controlled rebuild and compute-init is not supported, as these nodes will not be running slurmd.
  Security groups are those from opentofu variable nonlogin_security_groups by default, but may be overriden.

• Also adds compute nodes into an inventory group of the same name as the note group, in addition to the existing ${cluster_name}_${group_name} inventory group required for stackhpc.openhpc role partition configuration. This simplifies multi-environment configuration.

[sjpb]

Note this is exactly as per tofu/login.tf, except it has non-login secgroups by default, and those can optionally be overriden.

[bertiethorpe]

LGTM, once CI passing. I agree that login and compute should be suffixed with "_nodegroups" but we can live with it. It would be good to test this in CI at some point too

[sjpb]

Should have said, this has been tested locally using the CI environment, so it does actually work! Bit loath to add it to CI as we already struggle with resources.