Introduce NameServer interface and add default Name Server implementatio...

Currently NameServer by default is used in Bridge JobDriver and starts to run when it is constructed. If clients don’t bind any implementation, by default they will still get Name Server. 

This PR is to introduce DefaultNameServerImpl so that if clients don’t merge NameServer configuration into driver configuration, the default server would do nothing.  

This PR also created an interface for NameServer and renamed original NameServer to NameServerImpl. 

Test cases are updated accordingly. 

This addresses the issue REEF-53 https://issues.apache.org/jira/i#browse/REEF-53?filter=-1