fix(discovery): improve MQTT discovery for scene CC

[ccutrer]

• don't expose writable values as read-only scenes
• give HA a hint when values are numbers and not strings

[coveralls]

Pull Request Test Coverage Report for Build 7608418678

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• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.006%) to 22.151%

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Totals
Change from base Build 7608408616:	-0.006%
Covered Lines:	3764
Relevant Lines:	18066

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💛 - Coveralls

[robertsLando]

LGTM