Load a private key and the corresponding certificate. The certfile
        string must be the path to a single file in PEM format containing the
        certificate as well as any number of CA certificates needed to
        establish the certificate's authenticity. The keyfile string, if
        present, must point to a file containing the private key. Otherwise
        the private key will be taken from certfile as well.

def load_cert_chain(self, certfile, keyfile=None):
        """Load a private key and the corresponding certificate. The certfile
        string must be the path to a single file in PEM format containing the
        certificate as well as any number of CA certificates needed to
        establish the certificate's authenticity. The keyfile string, if
        present, must point to a file containing the private key. Otherwise
        the private key will be taken from certfile as well.
        """
        self._certfile = certfile
        self._keyfile = keyfile