adding default HttpServerImpl

Currently the default HttpServer is HttpServerImpl that creates Jetty server and starts it. If the clients doesn’t bind any implementation, by default they will still get Jetty server running in the JobDriver. We would like to provide an option for clients to choose not to have HttpServer if they don’t need it for performance consideration for example. 

DefaultHttpServer is introduced in the PR. If the client doesn’t bind Http configuration, the default behavior would be no Http server running in the JobDriver. 

Test cases are added for injecting HttpServer with DefaultHttpServer.

This is to address issue https://issues.apache.org/jira/i#browse/REEF-52?filter=-1