Enable setting per-compute group availability zones

[sjpb]

NB: This won't work as is, the availability zone host needs to be the node UUID, not the node name.

Adds a new option availability_zone_prefix to the compute groups in the skeleton terraform. If set, this is prefixed to the last portion of each compute node name split on "-" and used to set the instance availability zone

This enables scheduling baremetal instances to specific hosts. For example if openstack baremetal node list shows baremetal nodes compute-[0,5] then the following OpenTofu configuration will result in instances ${cluster_name}-hpc-[0,2] landing on the first 3x baremetal nodes. This can simplify troubleshooting.

# environments/$env/terraform/terraform.tfvars:
...
compute = {
  baremetal = {
   flavor = "baremetal.xlarge"
   availability_zone_prefix = "nova::compute-"
   nodes = ["hpc-0", "hpc-1", "hpc-2"]
  }
}
...