Relies on https://cloud-images.ubuntu.com being served by a cert signed
by one of the CA's trusted by the build host.
Change-Id: I690b755acca54789110c2c8fa723c8b87b2485c9
This is a necessary but not complete step towards supporting Fedora and Suse
distributions. Further work is needed (e.g. to quiesce daemons on
installation).
Change-Id: If3ea6093d41a21de755db52328226b84b5a3ede6