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...