Support mapping to compute + login to ironic nodes

[sjpb]

Add support to non-caas environments for mapping baremetal compute and/or login nodes to ironic hosts with the same names, e.g:

# environments/$env/terraform/terraform.tfvars:
...
cluster_name = "mycloud"
login_nodes = {
    login-0 = {
        flavor: "baremetal.highmem"
        match_ironic_node: true
    }
}
compute = {
    hpc = {
        flavor = "baremetal.xlarge"
        match_ironic_node: true
        nodes = ["hpc-0", "hpc-1", "hpc-2"]
    }
}
...

will result in an instance named mycloud-login-0 on ironic node login-0, and compute instances mycloud-hpc-N on ironic nodes hpc-N.

NB: To support this the definition of login nodes has changed slightly.

Also fixes a reversion where with the tf variable volume_backed_instances set, compute nodes were not actually volume-backed.

Note this PR only affects new environments, unless changes from skeleton terraform are copied over to site-specific environments.

Tests:

• Works for non-mapped nodes on a cloud where user does not have support for listing baremetal nodes
• Works for mapping nodes on a cloud where user does have support for listing baremetal nodes
• Can create an entire volume-backed cluster