This addresses previously-closed issue #2876, which points out that instances created with the `--google-use-internal-ip` command-line flag are still assigned an external IP address. The new flag (which implies the presence of `--google-use-internal-ip` if it isn't specified) will cause the new instance to have no externally-accessible IP address. Signed-off-by: Jason Roehm <jroehm@gmail.com>