Da steht doch genau was du machen musst, wenn du einen bereits konfigurierten MySQL Server hast:
ZitatIf you are installing Plesk on an already configured MySQL server, you need to specify the administrator's credentials to succeed with the installation. To do this, you need to create a file - /root/.my.cnf with the 'client' section where you need to provide user and its password ("user = $admin_name" and "password = $admin_pass"). After installation is finished, the file /root/.my.cnf will be renamed to /root/.my.cnf.bak