Add unique_id to MQTT Light

[bieniu]

Description:

Add unique_id to MQTT Light

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#6121

Example entry for configuration.yaml (if applicable):

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

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:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[houndci-bot]

blank line contains whitespace

[fabaff]

Thanks 🐦