CoreOS Stable imagetype

This adds the CoreOS imagetype...

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/

**NOTE:** Joyent does not support CoreOS