Add the `gce` image type intended for Google Compute Engine. The image
is BYOS - bring your own subscription and requires registering in order
to access Red Hat content.
Signed-off-by: Tomas Hozza <thozza@redhat.com>
Nightly composes are more stable because they contain only builds attached
to advisories whereas development composes contain latest brew builds.
Use nightlies because they should be stabler.
Signed-off-by: Ondřej Budai <ondrej@budai.cz>
