Switch to using EurekaClientConfig to determine client zone

This makes perfect sense since there is already support in the API.
Unfortunately Eureka also uses that information to set the Server
zones by default (duh?!) - see DiscoveryClient.getZone(InstanceInfo),
so unless you aggressively update them, the zones will all be the same
in all servers.

I fixed that problem by unconditionally applying the "domain as zone"
guess algorithm, which is fine unless you actually want to use
the Amazon metadata. So that's an outstanding problem

See gh-30