CONFIG_SCHEMA is missing payload defaults in digital_outputs node

[x] I have read the README.md file and visited the Github Wiki to see if the information there helps.

Describe the bug Runtime error occurs in logging message if payload_on and payload_off are not configured for digital outputs

Expected behavior "ON" and "OFF" should be used as default payload for digital outputs when nothing is configured.

Error messages and traceback

(ERROR): Exception while handling received MQTT message:
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/pi_mqtt_gpio/server.py", line 528, in on_msg
    handle_set(topic_prefix, msg)
  File "/usr/local/lib/python2.7/dist-packages/pi_mqtt_gpio/server.py", line 221, in handle_set
    if payload not in (output_config["on_payload"], output_config["off_payload"]):
KeyError: 'on_payload'

Config

  host: mqtt
  port: 1883
  client_id: pi-mqttXXX
  user: ""
  password: ""
  topic_prefix: /oh3/

gpio_modules:
  - name: piface
    module: piface2

digital_inputs:
  - name: carport_contact
    module: piface
    pin: 7

digital_outputs:
  - name: carport_close_trigger
    module: piface
    pin: 7

Hardware

Platform: Raspberry Pi
Connected hardware: piface2

System:

OS: Raspbian
Python version: Python 2.7.9
User you're running as: root
Using a virtualenv?: no

Additional context I think there are simply two lines missing in init.py: default "ON" between 264/265 default "OFF" between 268/269

flyte / mqtt-io

CONFIG_SCHEMA is missing payload defaults in digital_outputs node #159