MQTT: take C part away from python code for readability.
C lines change a bit because of unnecessary escaping for '\'.
<?xml version='1.0' encoding='utf-8'?>
<OPCUAClient xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<AuthType>
<x509 Certificate="my_cert.der" PrivateKey="my_private_key.pem">
<Policy/>
<Mode/>
</x509>
</AuthType>
</OPCUAClient>