When the parent (bootstrap) context registers the app as well as consuming
the registry it causes problems, e.g. when you refresh that context is
re-created and destroyed, which tends to unregister the app and it's hard
to get it back. It is simpler if the parent context use eureka read-only.
To ensure this we have to remove the @EnableDiscoveryClient from the
bootstrap context.

Tested with vanilla config server and eureka with the 'eureka-first'
sample and the 'zuul-proxy-eureka' sample.

Fixes gh-421