We now have a prebuilt tarball that we download from boss, that
has sshd, cloud-guest-utils, etc, preinstalled, to save time.  Use
it by default.

Also, create (at least) two images by default: trusty-server and
trusty-server-multi-nic (which has support for up to 8 virtual NICs).
Turns out Ubuntu distros have horrible support for "variable" NICs;
if the NIC is listed in /etc/network/interfaces, the setup scripts
will refuse to generate the right upstart event, which hangs the
cloud-guest-utils init script for a long, long time.  So I've hacked
a little service together that notes nonexistent NICs in
/etc/network/interfaces and fires a fake complete event at the right
time.  I didn't move this hack into the default image because I don't
trust it enough.

Also, create trusty-server first, so the first image is sane.