Also fixes a bug where the test when no payload is given succeeds only
because we run the jinja templating out of memory. Sending an empty
payload instead of trying to fix it through validation for now.
* Service validation for MQTT component.
* Fixes for failing mqtt tests.
- Changed behaviour when both 'payload' and 'payload_template' are provided
instead of silently ignoring the payload_template value.
- Have validation accept any type of payload and leave encoding to paho-mqtt.
