Use validated types for configuration

[qmonnet]

I'm not sure this is the right approach; this looks like a lot of data cloning for not too much gain.

Fixes: #554

TO DO:

• Use validated types for .device and .underlay in ValidatedExternalConfig
• Commit titles + descriptions

[Fredi-raspall]

I'm not sure this is the right approach; this looks like a lot of data cloning for not too much gain.

I think I would wait until the new API is in place, esp. if you comment that the gain is minor?

Raising this because Vpcs and Peering objects may soon be gone.
So, a better exercise would be to see how we model the pieces of information that particularly NAT requires in preparation for such a change?

+cc @mvachhar