Since there is no Heartbeat or InstanceRegisteredEvent when using the
SimpleDiscoveryClient we need to listen on the ApplicationReadyEvent
and register the configured instances

closes #505

A new property `spring.boot.admin.discovery.services` is added. It can
be used to restrict the services registered via discovery to a
whitelist (supporting simple patterns). Until now only blacklistin was
supported. The default whitelist permits all services.

fixes #395

fixes #387

With this commit you can use simple patterns to match services to
ignore.
A pattern can look like this "foo*", "*foo", "foo*bar".

closes #377

With this change it is possible to include credentials in the instance
metadata which will then be used to access the client endpoints using
HTTP Basic authorization.

closes #359