nixos-iso: Use a simpler disk label

When formatting a nixos usb using my windows machine I noticed that the
disk labeling using periods was not compatible with my linux kernel /
udev recognition. When labeling a volume NIXOS_14.10 under Windows, it would
show up as NIXOS_14_10 on NixOS. This meant that /dev/root would never come
up at boot time, preventing the livecd from starting.

This patch works around this issue by eliminating any special characters
other than underscores. The previous versioning didn't seem all that
useful, especially when consdering there are many different version of
the year.month named iso.