Make disk size configurable per image

Merged Kristian Klausen requested to merge klausenbusk/arch-boxes:configurable-disk-size into master
Shrink the cloud image to the bare minimum (2GB)

cloud-init resizes the disk[1] so we don't need a "big disk" with a lot
of unused space + some cloud providers charges for the unused space +
some cloud providers offers VM with disk < 20GB.


Fix #127
Expand the basic disk image from 20G -> 40G

It is meant for local usage so the disk should be "big enough".
Edited by Kristian Klausen

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