CDAP-663 Implemented healthcheck for router and auth-server.

The PR enables health check for router and auth servers independent of downstream services.  
This may work now, but seems like a problem waiting to happen for future code changes.  It's creating a special path that circumvents security.

Can't we do the service check handling as a new HttpHandler that sits higher in the pipeline than SecurityAuthenticationHttpHandler?  If it's a status request, then it just returns the response instead of letting the request pass through.