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

smilart/scripts,smilart/scripts,endocode/scripts,vmware/coreos-scripts,nicescale/scripts,fivethreeo/scripts,marineam/coreos-scripts,coreos/scripts,sigma/coreos-scripts,zhang0137/scripts,crawford/scripts,andrejro/coreos--scripts,mischief/scripts,marineam/coreos-scripts,mjg59/scripts,andrejro/coreos--scripts,smilart/scripts,glevand/coreos--scripts,coreos/scripts,mjg59/scripts,BugRoger/scripts,endocode/scripts,crawford/scripts,trnubo/scripts,crawford/scripts,nicescale/scripts,trnubo/scripts,gtank/scripts,mischief/scripts,andrejro/coreos--scripts,trnubo/scripts,fivethreeo/scripts,gtank/scripts,endocode/scripts,gtank/scripts,BugRoger/scripts,vmware/coreos-scripts,mischief/scripts,glevand/coreos--scripts,BugRoger/scripts,vmware/coreos-scripts,mjg59/scripts,sigma/coreos-scripts,sigma/coreos-scripts,bcwaldon/coreos-scripts,fivethreeo/scripts,bcwaldon/coreos-scripts,zhang0137/scripts,marineam/coreos-scripts,nicescale/scripts,zhang0137/scripts,bcwaldon/coreos-scripts