Add new mobile_app webhook command: get_zones (#22604)

## Description:

Adds a new `mobile_app` webhook command, `get_zones`, which just returns all zones.

## 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.

homeassistant/components/zone/config_flow.py

@@ -14,7 +14,7 @@ from .const import CONF_PASSIVE, DOMAIN, HOME_ZONE
 
 @callback
 def configured_zones(hass):
-    """Return a set of the configured hosts."""
+    """Return a set of the configured zones."""
     return set((slugify(entry.data[CONF_NAME])) for
                entry in hass.config_entries.async_entries(DOMAIN))