For BGP neighbor information (determine if address family is not negotiated)

[mirceaulinic]

Decide how to handle this case:

Now, we probably want to signal if an AF is negotiated or not. We have 
two solutions IMHO:
a. Easiest would be to not return it at all. After all, get_bgp_neighbors 
returns state, not configuration.
b. Alternatively, we could also discuss adding a capabilities field where 
we list which capabilities were negotiated successfully.

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Issue moved from napalm-automation/napalm-ios#156