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>
(cherry picked from commit 4a664ab4f91bdd226212e106957202ca45b20fa7)