Use custom DataCenterInfo to make instance ID unique

The problem manifests itself as errors in the Eureka server
log (gh-47), but originates in the client because it is sending
requests to /eureka/apps/{NAME}/{ID} with the wrong ID. The
InstanceInfo has an ID that is derived (for preference) from
the EurekaInstanceConfig.dataCenterInfo, so that's the best
way to fix it.

See gh-63, Fixes gh-47