Force-unmount anything that's mounted

mountmedia became very aggressive and insists on mounting
any ext2/3/4 partitions on local disk (say, the old /boot).
partman does not let you preseed an answer to the "do you want
to unmount any mounted disks?" question.

So we resort to the early_command to iterate over all extant
partitions and unmount them if they're mounted
Default to trusty

Default to trusty for the PXE install