ipdevinfo doesn't list aggregators for some Juniper interfaces

[lunkwill42]

It appears Juniper devices will report multiple aggregator ports for some aggregated interfaces. These seem to be largely various logical subunits of the main aggregator interface.

The codebase is not equipped to handle this well, even though NAV's data model is.

A simple heuristic here would be to have Interface.get_aggregator() only return the interface with the lowest ifIndex, assuming this is the lowest level interface.

A more complex heuristic would examine the interface relationships in ifStackTable to decide whether they are logically connected or not.

Another more complex, and probably undesirable heuristic, would be to allow for multiple aggregators, and also to report linkDegraded for all of them when an aggregated interface link loss is detected.