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

The PR enables health check for router and auth servers independent of downstream services.  
Why need to catch and re-throw? The method already has `throws Exception`.

As a side note, whenever you catch and re-throw, always include the original exception when constructing the new one, otherwise there will be no stack-trace on the real problem, making it hard to debug in general.