Checkpoint commit for OPENDJ-1235 : Migrate configuration framework
* Refactor server classes to remove ConfigEntry class
** Move listener registration from ConfigEntry to ConfigurationRepository
** Use DN to register or retrieve listeners
** Use Entry in ConfigXXXListener classes
* Refactor server classes to remove ServerManagementContext singleton
** Server context is passed as argument in constructors
* Migrate test cases from org.opends.server.admin.server package
** Rewrite ListenerTest class to test listeners without server start, using mocks
** Other test cases are not working yet