Comment 9 for bug 1286818

In actuality, apt does *not* support fallback to additional repositories for the same packages, confirmed myself by testing in various constructed failure scenarios. Bug 601916 has been open against apt for several years asking for it to be correctly implemented.

For now I'm experimenting with https://review.openstack.org/107060 in an attempt to implement a hackish fallback mechanism into DevStack itself, but long term the best solution will be maintaining local validated mirrors in each provider.