cloud: switch imagebuild for GCP image to tar-gz format
    
    It turns out koji doesn't know about `gce` even though ImageFactory
    does. Really all we need is the output disk image to be a tar.gz so
    let's try with the `tar-gz` format in koji, which looks like it was
    intended for use with GCP.
    
    Signed-off-by: Dusty Mabe <dusty@dustymabe.com>