DATAMONGO-789 - Support login via different (e.g. admin) authentication database.

MongoDbUtils now supports to perform the authentication against a dedicated authenticationDatabase - if no authenticationDatabase is given explicitly then the given regular database will be used. The authentication database can be configured via the "authentication-dbname" attribute of the db-factory element in xml config or by overriding the getAuthenticationDatabaseName() method of AbstractMongoConfiguration.