core: cover.mqtt no longer allowing device_class

Home Assistant release with the issue:

0.87.0

Last working Home Assistant release (if known): 0.86.4

Operating environment (Hass.io/Docker/Windows/etc.):

Docker

Component/platform:

cover.mqtt

Description of problem: After updating to 0.87.0 I get errors on startup staying that ‘device_class’ is an invalid option for cover.mqtt.

Cover documentation still lists it as a valid option: https://www.home-assistant.io/components/cover/

Problem-relevant configuration.yaml entries and (fill out even if it seems unimportant):

#cover:
  - platform: zwave

  - platform: mqtt
    name: "Master Bedroom Blinds"
    command_topic: "masterbedroom/blinds/command"
    tilt_command_topic: "masterbedroom/blinds/tilt"
    tilt_status_topic: "masterbedroom/blinds/status"
    tilt_closed_value: 0
    tilt_opened_value: 60
    device_class: 'window'
    tilt_min: 0
    tilt_max: 100
    optimistic: false

Traceback (if applicable):

2019-02-07 22:59:38 ERROR (MainThread) [homeassistant.config] Invalid config for [cover.mqtt]: [device_class] is an invalid option for [cover.mqtt]. Check: cover.mqtt->device_class. (See /config/covers.yaml, line 3). Please check the docs at https://home-assistant.io/components/cover.mqtt/
2019-02-07 22:59:38 ERROR (MainThread) [homeassistant.config] Invalid config for [cover.mqtt]: [device_class] is an invalid option for [cover.mqtt]. Check: cover.mqtt->device_class. (See /config/covers.yaml, line 15). Please check the docs at https://home-assistant.io/components/cover.mqtt/
2019-02-07 22:59:38 ERROR (MainThread) [homeassistant.config] Invalid config for [cover.mqtt]: [device_class] is an invalid option for [cover.mqtt]. Check: cover.mqtt->device_class. (See /config/covers.yaml, line 27). Please check the docs at https://home-assistant.io/components/cover.mqtt/

Additional information: