Fix debian-sys-maint grant for installs with moved datadir

I actually introduced this issue, but I was previously moving the data to its new location rather than using the built-in approach to create new data, so I hadn't run into it.

If we have set a `server.datadir` other than `/var/lib/mysql`, the `debian-sys-maint` user won't already exist in the newly-created database. That means that the password set will fail.  Moving the password after the grants fixes that.