better subvariants for cloud images
    
    Vagrant is an imagetype, it's not properly a component of the
    subvariant; both the raw disk and vagrant cloud_base images
    should have the same subvariant.
    
    The Docker image's subvariant should be Docker_Base to stay in
    line with the Cloud images, I think.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>