Ability to respond to variable queries over MQTT

[bblacey]

It would be good to extend the SensorMqtt plugin to include a relatively benign subscription as a "request/response" proof of concept for controlling a Vera remotely over MQTT. Other adventurous souls could expand the set of actions to address more use cases.

Concept

Implement a relatively benign topic action that allows a remote mqtt client to request an update for one or more service variables.

Design

• If a service variable request includes device id, then the variable will only be returned for that device.
• If a service variable request does not specify a device, then the variable will be returned to all devices that provide the requested service variable.
• The service variable requests are encoded as a JSON array allowing the remote client to request one or more variables in a single request

Service Variable Request Topic

/Vera/(SerialNumber)/GETVARIABLE

Payload Example

{
    [
      {"ServiceId: "urn:upnp-org:serviceId:TemperatureSensor1", "Variable": "CurrentTemperature"},
      {"ServiceId: "urn:upnp-org:serviceId:HumiditySensor1", "Variable": "CurrentLevel", "DeviceId": 49}
   ]
}

• The first element requests the current temperature from all TemperatureSensor devices.
• The second element requests the current humidity from device 49