roles/mosquitto: Add role to deploy MQTT server

*Mosquitto* implements an MQTT server.  It is the recommended
implementation for using MQTT with Home Assistant.

I have added this role to deploy Mosquitto on the Home Assistant server.
It will be used to send data from custom sensors, such as the
temperature/pressure/humidity sensor connected to the living room wall
display.

roles/mosquitto/handlers/main.yml

@@ -0,0 +1,4 @@
+- name: restart mosquitto
+  service:
+    name: mosquitto
+    state: restarted