Wifi managers improvements (#503)

* Add parameters to wifi manager: gateway name and base topic
Enable to restart the ESP and erase the parameters by MQTT

docs/use/gateway.md

@@ -0,0 +1,9 @@
+# System commands (ESP only)
+
+## Restart the ESP
+
+`mosquitto_pub -t "home/OpenMQTTGateway/commands/MQTTtoSYS/set" -m '{"cmd":"restart"}'`
+
+## Erase the ESP settings
+
+`mosquitto_pub -t "home/OpenMQTTGateway/commands/MQTTtoSYS/set" -m '{"cmd":"erase"}'`