add script shortcut for activating scenes

[mdonoughe]

Description:

Use - scene: scene.<scene name> in a script to activate a scene instead of writing

- service: scene.turn_on
  data:
    entity_id: scene.<scene name>

Should it be just -scene: <scene name>? Writing -scene: scene. seems redundant.

Related issue (if applicable): fixes #27026

Pull request with documentation for home-assistant.io: home-assistant/home-assistant.io#10616
Pull request for frontend: home-assistant/home-assistant-polymer#

Example entry for configuration.yaml (if applicable):

# Example configuration.yaml entry
script:
  message_temperature:
    sequence:
      - scene: scene.morning_living_room

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.
• 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 does not interact with devices:

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

[balloob]

Let's validate it as entity ID for the scene domain.

Suggested change

	_SCRIPT_SCENE_SCHEMA = vol.Schema({vol.Required("scene"): string})
	_SCRIPT_SCENE_SCHEMA = vol.Schema({vol.Required("scene"): entity_domain("scene")})

[balloob]

Awesome! This is a great contribution 🎉 🐬

[balloob]

I think that we should keep it as an entity ID and not object ID, because we don't use just object ID anywhere in the codebase.