RackNetworkConfig: Allow Ipv6

[jgallagher]

#3399 changed a handful of RackNetworkConfig field types from String to Ipv4Addr (based on their current usage, they must be IPv4 and cannot be IPv6). While reviewing, @internet-diglett noted that that restriction should be lifted: we should allow IpAddr (for either v4 or v6) instead.

[rcgoodfellow]

I've run into this when plumbing support for BGP. This is inconsistent with the switch port settings API, which allows setting uplinks for either address family. We should fix this as soon as possible as it's going to become harder and harder to unwind.