kola/tests/ignition: fix TFTP test on Packet
kola/tests/ignition: fix TFTP test on Packet

On Packet, Ignition runs before the machine's private IP has been
configured. On that platform only, contact the TFTP server via its
public IP.

dm0-/mantle,glevand/coreos--mantle,dm0-/mantle,glevand/coreos--mantle