Network extensions

@icecrime 
See comment above regarding endpoint name. Another potential issue I could imagine here would be the case where a single sandbox joins multiple networks managed by the `simplebridge` driver under the same name. If a sandbox links to networks `A` and `B` (both managed by the `simplebridge` driver) under the same name `"sad_einstein"`, you'll have conflicting interfaces names. However that use case may be irrelevant, I don't know.