feature(gce_image): support passing user-data
feature(gce_image): support passing user-data

Now that we have GCE formal image, those support using user-data
to make sure we don't start scylla at boot

scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests