Invalid default behavior of `nxc start` for g5k-ramdisk flavour
As of fc79c9b9, nxc start -f g5k-ramdisk
(without additional parameters) tries to use invalid ip addresses by default (it seems to use the addresses for qemu instead).
An error such as The '--machines-file' option is mandatory for flavour g5k-ramdisk
would be clearer IMO.