Allow fixed ip lists to be longer than nodes list

[sjpb]

Moving baremetal nodes between e.g. production and staging is often necessary to test changes due to the limted number of them available. If these have fixed IPs specified via ip_addresses this change allows the IPs to be left while removing the actual nodes, which makes this move easier as the IP configuration can remain in source.

Also allows e.g. predefining expected IPs before fully expanding a cluster.

E.g. with this PR this configuration passes validation but creates no nodes:

# environments/staging/tofu/main.tf:
module "cluster" {
  ...
  compute = {
    gpu = {
      nodes = []
      ip_addresses = {
        mynet = ["192.168.0.14", "192.168.0.15"]
      }
    }
  }
}

[bertiethorpe]

LGTM