Ensure that Flocker release artifacts remain accessible

[Richard Wall]

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.