feat(disk_layout): follow new CoreOS partition layout

As outlined here we need a new partition layout, this patch makes the
necessary changes:
https://groups.google.com/forum/#!topic/coreos-dev/bA7gwGGoTng

The first big change is making all of the scripts obey partition numbers
based on labels in the disk_layout.json. This makes it much easier to
change later on.

The second big change is in the layout itself. The json file was updated
to reflect the document above.

And finally the grub boot configuration needed for pv-grub and pygrub
were added to the create_legacy_bootloader_templates.sh library utlity.

Everything seems to work and boot now.