Add support for configAttribute settable in iotile-gateway

For gateway applications, we want to be able to change their configuration from the cloud asynchronously. The functional flow using the example of mqtt-forwarder might look something like this:

(1) Check the cloud for a configAttribute pending update for the gateway's applications
(2) Identify that an update is pending for the mqtt-forwarder
(2) Download the configAttribute for mqtt-forwarder
(3) Apply the update
(4) Restart (if necessary) the mqtt-forwarder
(5) Inform the cloud that an update was applied

The cloud will then verify that the update was applied correctly, if necessary.