fix(prod_image_util): restore /etc/gentoo-release

Vagrant reads this file to determine that we are CoreOS... so lets not
break that just yet. A PR to switch to os-release has been posted:
https://github.com/mitchellh/vagrant/pull/2985

Some day gentoo-release will be dropped but that day is not today.
Merge pull request #178 from marineam/gentoo-release

fix(prod_image_util): restore /etc/gentoo-release