This should fix this issue permanently
touch /etc/munin/plugin-conf.d/mysql.conf
echo -e "[mysql*]\nuser root\ngroup wheel\nenv.mysqladmin /usr/bin/mysqladmin\nenv.mysqlopts --defaults-extra-file=/root/.my.cnf" > /etc/munin/plugin-conf.d/mysql.conf
/etc/init.d/munin-node restart
