This cleanly refactors everything we do to VM images (asserting the
random passwd, disabling root passwd, changing sshd config, etc).  This
allows us to support adding extra images based on URL/name user provides
us in params, and to allow them to call our script after profile
instantiation to add an image.  It's fairly comprehensive and it
certainly works for the common cloud images from various linux vendors.

It also rolls multi-nic support into each image.  We do this via
boot-time udev scripts and dhcp hooks that ensure we don't add routes
for interfaces other than eth0 (this ensures that the default gateway is
always attached to eth0).  The old, hacky, sometimes-broken multi-nic
support is gone, as is the special image.  I have no idea why cloud
images don't just include this feature by default... it's not hard at
all.

We support Ubuntu and Fedora/Centos.  We support basically the image
formats that qemu-nbd supports (i.e., qcow, vmware, vdi, raw), and gz or
xz compression.  That seems to cover the core spectrum.

On aarch64, we yank the kernel and initrd out of the image's /boot and
create an AMI/AKI/ARI image tuple, instead of uploading the raw disk
image.  I have never figured out how to boot a raw Ubuntu cloud image on
KVM/aarch64, and the HP guys never got back to me.  So this is the only
way I know (well, there's UEFI, and there's a UEFI aarch64 BIOS, so the
UEFI cloud images might work... but life is way too short for all that
fun).