Introduce the notion of cluster identifier

The members of a cluster do not have the notion of unique identifiers (a unique id for each member of the cluster, stable over time, especially after restart).
Having this information will help solve several technical issues. For instance, by allowing to persist the member of the cluster responsible for a task.

 This improvement involves adding a RemoteObservationManagerConfiguration#getId() operation as well as at the initialization of a file containing the UUID used as identifier on each cluster members if it does not exist.