Extract Eureka Dashboard to independent module

Hello,

I propose in this PR to extract eureka dashboard in specific module.
This can be usefull when we don't want the webjars dependency imported to eureka server application or if we don't want dashboard anymore.

In my case I develop a [spring-cloud-dashboard](https://github.com/VanRoy/spring-cloud-dashboard) that mix Eureka, Hystrix and Spring Boot Admin dashboards, and I would like disable standard eureka dashboard.

If you are interested to this PR, I can help you to add starter, change doc and samples to match with this new module.

Thanks.
Julien.