Customize live image to automatically fetch authorized keys

[wmedlar]

Pulling down keys from github automatically in the live image will save me from having to pull out the monitor and the keyboard each time, making it a lot easier to swap the USB stick around between all the boxes I'm collecting.

I think we can do this with a cloud-config script and stash it in the live image.

[wmedlar]

Some notes:

• iso links: https://coreos.com/os/docs/latest/booting-with-iso.html
• brew install squashfs for tools for dealing with image's squashfs
• coreos filesystem is stored on the image in a gzip'd cpio file (coreos/cpio.gz) which unpacks (gzip -dc /Volumes/CDROM/coreos/cpio.gz | cpio -idm -) into an empty etc directory and a usr.squashfs which in turn unpacks (unsquashfs usr.squashfs) into the full usr filesystem (how to repack?)
• usr/share/oem/cloud-config.yml is probably the right place to stash our script (does it need the yml extension?)