DATAMONGO-628 - Allow dependency of config layer to GridFS.

Updated Sonargraph architecture description to allow a dependency form the configuration layer to the GridFS layer. Dependency was introduced by c5a99b5b5e703bfa3948def48d8f7c8b1b69f20e.
DATAMONGO-641 - Reformatting to prepare fix.
DATAMONGO-641 - Fixed potential NullPointerException in MongoLog4jAppender.

Log4j appender now only closes Mongo instance if available.