Remove default class for HttpServer interface

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. It will be covered in a separate PR. 

This PR is to remove default class from interface HttpServer. If HttpServer is not defined in Driver constructor, and if the client doesn’t bind Http configuration, then there should be no HttpServer running. 

This is to address issue https://issues.apache.org/jira/i#browse/REEF-52?filter=-1