mysql handler using defaults-extra-file lets /root/.my.cnf override credentials

In a really weird situation today where root didn't have any special permissions on mysql (I believe it was intentional, although really weird) the mysql handler was not able to perform dumps of databases other than those owned by the root user.

This was happening because --defaults-extra-file is evaluated before /root/.my.cnf and so the file in root's homedir was overriding credentials in /etc/mysql/debian.cnf.

I'd argue for the handler to switch to using --defaults-file instead of --defaults-extra-file which doesn't suffer from the same overriding potential, although I don't know if there was a reason in the first place to use the argument with the -extra variant.