Decide the source of the base images

[Jean-Philippe Evrard]

We have decided to use lxc images from certain urls, but it's not documented.

We also have URLs that aren't stable, so we should:

- Document the source of the image, and why we chose the image in the reference architecture.
- Make sure the image URL is consistent.

[Jesse Pretorius (jesse-pretorius)]

We had the opensuse URL change today and had to do this emergency patch to stop all our tests failing: https://review.openstack.org/562599

Whatever we do, we need a stable interface that works.

LXC has a buggy DNS resolution, but it has worked reasonably well.
LXC's images need quite a bit of adjusting (package removals and such) to use.

We could bite the bullet and implement a playbook to use the ansible-role-diskimage-builder role to prepare the base image, breaking it out of the lxc_hosts/nspawn_hosts roles. This would also allow us to unify the trove/octavia diskimage-build processes. I expect the base image build will be very quick given that we require very few things in the image. I'll see if I can find some time to prototype it.