remove password info from maxscale log

When adding "log_info = 1" to maxscale.cnf, a lot of "Added user:" lines appear in the log file, that contain each user created, including its password hash.

info   : Added user: INSERT OR REPLACE INTO mysqlauth_users VALUES ('user', 'host', NULL, 0, 'password-hash')

info   : Added user: INSERT OR REPLACE INTO mysqlauth_users VALUES ('user', 'host', NULL, 0, 'password-hash')

This is to request for passwords must not be stored in the logs at "Info" level.