[ocp 4.18] Butane should not suggest unsupported layouts in error message

[travier]

#484 added new layouts but changed the error message that is common to all config variants: 0d2fa0f#diff-f4ab90dff151f635ae66322acea878c8b9027eb78d5c69738290ad51a050479aR54-R57

This makes 4.18 config failures suggest invalid layouts:

$ cat ocp418.bu
variant: openshift
version: 4.18.0
metadata:
  name: foo
  labels:
    machineconfiguration.openshift.io/role: worker
boot_device:
  layout: s390x-eckd
  luks:
    device: /dev/dasda
    tang:
      - url: https://tang.example.com
        thumbprint: REPLACE-THIS-WITH-YOUR-TANG-THUMBPRINT

$ ./bin/amd64/butane --pretty --strict ocp418.bu
warning at $.boot_device.luks.device, line 10 col 5: Unused key device
error at $.boot_device.layout, line 8 col 11: layout must be one of: aarch64, ppc64le, s390x-eckd, s390x-virt, s390x-zfcp, x86_64
Error translating config: source config is invalid

We thus likely need to split this error message in two, one for before the new layouts, one for after.