[LOG4J2-358] NoSQLAppender using MongoDB provider ignores username and password attributes

Added missing code to MongoDBProvider to actually use the username and password attributes.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1514958 13f79535-47bb-0310-9956-ffa450edef68