Exposes the eureka.registration.enabled property for eureka-server

Eureka instances attempt to register with other Eureka instances on startup.  However, in situations like development where you typically only have a single Eureka instance running this can be confusing behavior.

This pull request exposes the eureka.registration.enabled property, defaulted to true, to prevent this self registration in specific scenarios.

See this wiki page for more information: https://github.com/cfregly/fluxcapacitor/wiki/NetflixOSS-FAQ