Hostname is 'localhost'
You have to setup mySQL user accounts in CPanel. Be aware that the SQL server is currently undergoing some updates, so it is likely some problems may arise.
it will be localhost and you don't have to use your CPanel username here. Your password will be what you have given during the creation of MySQL database.
Indeed there were some issues with mySQL at the same time I tried to setup joomla but all went fine after.
I can confirm cpanel is not needed login is taken from the x10 account creation.