Added config validator for future group platforms (#12592)

* Added cv.EntitiesDoamin(domain) validator

* Check if all entities in string or list belong to domain
* Added tests

* Use factory function and entity_ids

* Different error message

* Typo

* Added entity_domain validator for a single entity_id

* Image_processing platform now uses cv.entity_domain for source validation

homeassistant/components/image_processing/__init__.py

@@ -43,7 +43,7 @@ DEFAULT_TIMEOUT = 10
 DEFAULT_CONFIDENCE = 80
 
 SOURCE_SCHEMA = vol.Schema({
-    vol.Required(CONF_ENTITY_ID): cv.entity_id,
+    vol.Required(CONF_ENTITY_ID): cv.entity_domain('camera'),
     vol.Optional(CONF_NAME): cv.string,
 })