gce: Introduce --gce-upload-dir option

This specifes a Google Cloud Storage directory to upload local image to.

$ capstan run -p gce --gce-upload-dir gs://my_bucket -i ./local.osv.image.tar.gz $instance_name

will upload the local OSv image to GCE and start a OSv instance.

Signed-off-by: Asias He <asias@cloudius-systems.com>