Ensure that Flocker release artifacts remain accessible

All past release artifacts are hosted on AWS S3 storage paid for by ClusterHQ.

Choose a store for which is free and which is likely to remain free for Flocker's useful lifetime.
Copy Centos and Ubuntu packages to the new store and set up package repositories
Update documentation to point users at these new repositories.
Update release process to explain how to upload new packages to the chosen store.

ClusterHQ / flocker