Make login optional

We're trying to develop an extension using pyvmomi, and extensions don't log in using username and password, but use the LoginExtensionByCertificate API. However, when we call Connect or SmartConnect, we must provide a username and password for log in... It would be great if the __Login function was called from Connect only if user and pwd were provided, so that it would be optional instead of mandatory. This way we can create a Connection class and then call the API on our own: service_instance.content.sessionManager.LoginExtensionByCertificate