Updated VirtualBox script for new releases
Updated VirtualBox script for new releases

Changes to handle stable, beta and alpha special versions.
Fixed to always import GPG key from official CoreOS web page.

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