This was kind of ugly, and caused a static usage of the eureka
DiscoveryCLient to become necessary again, just so the @Bean that is
provided for the user stays accurate (it has to be the one in the main
application context, even if the parent boot strap has different
instance metadata).

I tested with a vanilla Eureka server and config server, and used the
client in tests/eureka-first.

Fixes gh-268