MQTT cover component: Retain messages are not working.

[nunojusto]

Home Assistant release (hass --version):
0.28.2

Python release (python3 --version):
Python 3.4.2

Component/platform:
cover MQTT

Description of problem:
MQTT cover component: Retain messages are not working.
If i click the buttons on the home Assistant front-end it does not produce a retained message. Only send a normal message.

I've tested the switch mqtt component with a similar config and it does send a retained message.
So this must be a bug introduced when component code was migrated from rollershutter to cover.

Expected:
The MQTT messages to be retained

Problem-relevant configuration.yaml entries and steps to reproduce:

1- Create this cover
my config is

cover Blinds:
  - platform: mqtt
    name: "Room 1"
    command_topic: "blinds/control/room1"
    qos: 1
    retain: true
    payload_open: "UP"
    payload_stop: "STOP"
    payload_close: "DOWN"

2- execute mosquitto command to subscibe to all messages
mosquitto_sub -h 127.0.0.1 -v -t '#'

3- Click on the cover button (up, down, or stop)
4- The message appears on the mosquitto command, but if we CTRL+C the command and run again. It should appear again because the message was supposed to be retained. And it's not :(

Note: MQTT switch component works fine, so you can follow the behaviour configuring one MQTT switch

Additional info:

[MartinHjelmare]

I have a fix and will make a PR as soon as tests are written.

[nunojusto]

Great. Thx Martin

[MartinHjelmare]

The underlying cause was that the retain key had been duplicated in the voluptuous validation schema, when extending the schema, which is a bug in voluptuous:
alecthomas/voluptuous#219
alecthomas/voluptuous#192