Gathers a list of base URLs we will try.

def _GetBaseURLs(self):
    """Gathers a list of base URLs we will try."""
    result = config.CONFIG["Client.server_urls"]
    if not result:
      # Backwards compatibility - deduce server_urls from Client.control_urls.
      for control_url in config.CONFIG["Client.control_urls"]:
        result.append(posixpath.dirname(control_url) + "/")

    # Check the URLs for trailing /. This traps configuration errors.
    for url in result:
      if not url.endswith("/"):
        raise RuntimeError("Bad URL: %s URLs must end with /" % url)

    return result