Fix hang on systems with old F17 systemd (#910326)

In the future we may need to check package versions inside fedup, since
running fedup off the DVD isn't the same as *installing* it and running
it.

But since we don't do that yet, we don't need to worry about it yet.
move cachedir to __init__.py

We might want to use this in other parts of fedup, so...
refactor: split system prep stuff into fedup.sysprep

fedup.download was kind of crammed with stuff. This makes it only hold
the yum-based bits, which is nicer.
Add mdadm.conf to initrd if needed (#895805)

Check to see if the system needs mdadm.conf to boot, and if so, add it
to the initramfs.