mqtt_statestream: Update to append 'state' to topic for mqtt_discovery

[mw-white]

Description:

This patch updates the mqtt_statestream component to append /state at the end of the publish topic. After discussion in the forum of adding publishing of attributes and mqtt discovery data, it makes sense to make this change now before the component is published to avoid the need for a breaking change later if/when the other capabilities are added.

For example, given the base_topic of homeassistant, the state of an entity called light.master_bedroom would be published to homeassistant/light/master_bedroom/state. In the future, attributes would be published at this same level (ie. homeassistant/light/master_bedroom/brightness).

Related issue (if applicable): fixes #

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#3379

Example entry for configuration.yaml (if applicable):

mqtt_statestream:
  base_topic: homeassistant

Checklist:

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

• Documentation added/updated in home-assistant.github.io

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.

[balloob]

Thanks! 🐬