Fix mysensors overwriting gateway in GATEWAYS (#4013)

GATEWAYS was a dict, so would overwrite item if key was the same. This
would happen when using multiple MQTT gateways, since the device id is
the same (`mqtt`).

* Fix by changing GATEWAYS from dict into list.
* Use hass data to store mysensors gateways instead of having GATEWAYS
  be a global.

homeassistant/components/sensor/mysensors.py

@@ -20,7 +20,11 @@ def setup_platform(hass, config, add_devices, discovery_info=None):
     if discovery_info is None:
         return
 
-    for gateway in mysensors.GATEWAYS.values():
+    gateways = hass.data.get(mysensors.MYSENSORS_GATEWAYS)
+    if not gateways:
+        return
+
+    for gateway in gateways:
         # Define the S_TYPES and V_TYPES that the platform should handle as
         # states. Map them in a dict of lists.
         pres = gateway.const.Presentation