I think you should probably provide an interface/mechanism for setting the root user passwords in MySQL after the database is installed.
There are two root user entries for MySQL (localhost and mydomain.com) that don't have passwords initially.
I tried changing the root user password through Webmin, but it gave me an error:
DBI connect failed : Access denied for user 'root'@'localhost' (using password: NO)
It did appear to change the password, but just didn't recover nicely from this change. It probably should prompt you again for the log in user/password to access the database.
Anyway, this process seems to require a little more work.