Implement PXE fallback validation

[craiganderson]

For environments without a dedicated PXE interface (i.e., PXE over LACP trunk of primary NICs), validation that of PXE fallback would be ideal.

There may not be a way of automating this validation for all nodes (PXE-boot chicken-and-egg issue here). At a minimum it would be possible to perform an LACP-down test in genesis.sh to validate that a single interface can communicate without LACP for this one node.