Add default scopes if none are specified

[Temikus]

We probably should add the default bunch:

        "https://www.googleapis.com/auth/devstorage.read_only",
        "https://www.googleapis.com/auth/logging.write",
        "https://www.googleapis.com/auth/monitoring.write",
        "https://www.googleapis.com/auth/servicecontrol",
        "https://www.googleapis.com/auth/service.management.readonly",
        "https://www.googleapis.com/auth/trace.append"

, if scopes is empty. If it's set to an empty array, however, we should honor it.