This is a sample gist for building an image of Rackspace public cloud

template.json

{
    "builders": [
        {
            "type": "openstack",
            "username": "",
            "password": "",
            "provider": "rackspace-us",
            "region": "IAD",
            "ssh_username": "root",
            "image_name": "Test image",
            "source_image": "271e1090-77ad-4400-a1f7-73a922aca8e2",
            "flavor": "performance1-2"
        }
    ],
    "provisioners": [
    ]
}