Fix flash for esphome lights

[Sian-Lee-SA]

Fix for esphome lights to use the flash feature

Description:

Previously the flash function would not pass as the esp api client was expecting the attribute 'flash_length' and not 'flash'.

Checklist:

• [ x ] The code change is tested and works locally.
• [ x ] Local tests pass with tox. Your PR cannot be merged unless tests pass
• [ x ] There is no commented out code in this PR.
• [ x ] I have followed the development checklist

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

• Documentation added/updated in home-assistant.io

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

• The manifest file has all fields filled out correctly. Update and include derived files by running python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

If the code does not interact with devices:

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

[homeassistant]

Hi @CQoute,

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

Thanks!