Inter-domain traffic steering update

[ivanocerrato]

When the frog receives a graph update, it creates again the inter-domain connections.
For instance, if it selected GRE 0x25 at the time of deploying the graph, when such a graph is updated in selects a different GRE key to implement the same connection.

This behaviour derives from the fact that, in the past, the frog orchestrator didn't have state.
Instead now it stores information on the graphs, then, in case of update, it can reuse the same key previously used.