Juwang http server distributed shell command

HttpServer embedded in Reef now is able to get web request and distribute it to all the evaluators. This shows such an example.

The code contains a HttpServerShellCmdtHandler that implements HttpHandler and is bound to HttpServer through HttpHandlerConfiguration. When HttpServer receives a shell command request, the request will be redirect to HttpServerShellCmdtHandler, and then routed to ClientMessageHandler in HelloShellJobDriver, where it is distributed to all evaluators. 
This change also uses DriverServiceConfiguration for HttpServer related configurations to separate it from DriverConfiguration. 
Some more update at reef-webserver to support this function.