Added inner-class for admin client registration configuration which is
conditional on spring.boot.admin.url. Logfile endpoint is now available
when just using Spring Cloud Discovery.

The health-check is made at a fixed interval (default: 10s) for all expired status-information (default-lifetime: 30s).
The application health-endpoint must either respond with a JSON-Body having a status-field or with an empty body and all 2xx-http-responses are interpreted as UP and every other status as DOWN.

If `spring.boot.admin.client.useIpAdressOf=<network-interface>` is set properly the ip-address of the specified network-interface is used in the inferenced url to register with.
In case the address cannot be determined an exception is thrown and the client won't register.
This is for example useful in Docker environments making the inferenced urls work.

Checks for the presence of Ssl in ServerProperties and that it is
enabled, uses https in that case for client URL. Added test case for
option.

fixup! Add optional automatic deregistration on shutdown - turned off by default; only periodically try to register when the context is active

Add optional automatic deregistration on shutdown - turned off by default; only periodically try to register when the context is active

fixes #62

This change not only allows you to use `context-path` and `contextPath` but also respects automatic port assignment when using `server.port=0` or `managment.port=0`.
Also corrects the value if server.port != mangement.port and server.context-path and management.server-path is set.
fixes #57