mariadb: refer ubuntu/redhat to add mysql group

In ubuntu/redhat, we have:
...
ps -eo user,group,cmd | grep "mysql"
mysql    mysql    /usr/sbin/mysqld
...

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>