cluster_aws: don't start scylla-server after scylla-ami-setup
cluster_aws: don't start scylla-server after scylla-ami-setup

SCT will update scylla config after scylla-ami-setup and restart scylla-server,
let's pass `--stop-services` to ec2 userdata, then scylla-server won't start
automatically after scylla-ami-setup.

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

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