[Metal3] Add validation for the BMH object creation when provisioning network is disabled

When the provisioning network is disabled and the provisioning it's done through virtual media, i.e.:

apiVersion: metal3.io/v1alpha1
kind: Provisioning
metadata:
  name: provisioning-configuration
spec:
  disableVirtualMediaTLS: true
  preProvisioningOSDownloadURLs: {}
  provisioningNetwork: Disabled
  virtualMediaViaExternalNetwork: true

The creation of BareMetalHost objects using IPMI for bmc_address should not be allowed, as vMedia demands redfish-virtualmedia or idrac-virtualmedia.