Add PXE boot test
    
    This adds a whole wodge of stuff to support_server to make it
    act as a PXE server, then adds a new test which boots from PXE
    and so should hit the PXE server. We use the NFS install repo as
    that can be relied on to work for a support_server install.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>