Creating skeleton of driver configuration files

Client can submit a Job through REST API. For that, they need a serialized driver configuration file. In .Net/Java hybrid driver (see REEF-51 for a description of that), today the driver config file is created in Java JobClient, that is inconvenient for the clients.
 
This PR is to address this issue. It generates a skeleton of the driver configurations which only contains java configuration settings. This would leave flexibility to the client so that they can deserialize the driver configuration, and set additional parameters and then serialize the result into driver config file again.  

To make the configuration modularized, we also separate HttpServer configuration and NameServer configuration from driver configuration file when creating the skeletons. They would become building blocks. Client can choose what they want in the driver and merge them as need. This would make Http Server and Name server become optional instead of build-in in a .Net/Java hybrid driver. 

The driver skeleton configuration creation is built in pom file so that we can always get the latest config files on the fly. The config files generated are packed inside the jar file for clients to consume.

The PR added additional constructors for JobDriver (bridge) to make HttpServer and NameServer optional during the injection. 

This is for issue: https://issues.apache.org/jira/i#browse/REEF-54?filter=-1

Http configuration here is for HttpServerImpl and HttpReefEventHandler. Those two are provided by Reef in Java. .Net clients can bind their HttpEventHandlers. To make their HttpEventHandlers working, they need to use the HttpServer provided at Java side hence this HttpConfiguration. Long term, we can provide bridge for HttpServer for .Net to supply their own HttpServerImpl. For now, they leverage Jetty Server we build in Reef.  Therefor the configuration for it needs to be created at java side. 

In the original approach, HttpServer configuration is hard coded in Java JobClient and HttpServer is injected in Java JobDriver constructor. That means it is built in for bridge clients. As we discussed, here we just serialize the config files. It would be up to the .Net clients to merge it or not. 

Do I miss understand your question?