Configuration changed callback to Alignak

[mohierf]

The backend may implement a Web Service call back feature to keep Alignak informed about the configuration change.

Currently, the Alignak backend arbiter module periodicall checks if some items changed in the configuration. This is not really efficient because of the check period ... meaning that during several minutes a new host is not moditored by Alignak because not known!

If the backend was able to notifiy the alignak arbiter about any change (eg. new host in the backend), then the Alignak arbiter will reload and propagate a new configuration almost as soon as the host got created 😉