Adding CoreOS Stable images

AWS: https://coreos.com/docs/running-coreos/cloud-providers/ec2/
DigitalOcean: https://coreos.com/docs/running-coreos/cloud-providers/digitalocean/
Google: https://coreos.com/docs/running-coreos/cloud-providers/google-compute-engine/
Rackspace: https://coreos.com/docs/running-coreos/cloud-providers/rackspace/
Update to latest images for GCE/Rackspace
Merge pull request #22 from caskdata/feature/coreos-stable-imagetype

CoreOS Stable imagetype