light/mqqt_json: allow brightness scaling

[jkoelker]

Description:

Related issue (if applicable): fixes #

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#4403

Example entry for configuration.yaml (if applicable):

light:
  platform: mqtt_json
  name: test
  state_topic: "mqtt_json_light_1"
  command_topic: "mqtt_json_light_1/set"
  brightness: true
  brightness_scale: 4095

Checklist:

• The code change is tested and works locally.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.

[homeassistant]

Hi @jkoelker,

It seems you haven't yet signed a CLA. Please do so here.

Once you do that we will be able to review and accept this pull request.

Thanks!

[fabaff]

Looks good to me 🐦