Make Netatmo battery_percent icon dynamic

[notoriousbdg]

Description:

This change makes the icon for the Netatmo battery_percent sensors dynamic by setting device_class on the sensors instead of hard coding the icon.

Related issue (if applicable): fixes N/A

Pull request in home-assistant.io with documentation (if applicable): N/A

Example entry for configuration.yaml (if applicable):

netatmo:
  api_key: YOUR_CLIENT_ID
  secret_key: YOUR_CLIENT_SECRET
  username: YOUR_USERNAME
  password: YOUR_PASSWORD

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.