[sled-agent] Allocate VNICs from a per-sled etherstub device, rather than using the physical link

[smklein]

This has a bonus side-effect: It should let us remove the GZ-allocated addresses for routing, like those for the internal DNS service.

[smklein]

This issue seems to be blocking #718 for me.

Here's the situation I encountered:

• While hacking on [WIP] Use dns services instead of hard-coded IP addresses #1056 , I made RSS start CRDB, and provide DNS records in the internal-dns server.
• When Nexus started, it attempted to use a resolver pointing to the internal DNS server. This fails because there is no route from the non-GZ Nexus Zone to the internal DNS address. The Nexus' IP address is in a distinct subnet from the "reserved rack subnet", where the DNS service resides.

This is basically the same issue we ran into here, but it presumably applies to all non-GZ zones trying to access the DNS service.