Comment 1 for bug 716535

Just to clarify a few points about this:

- initially we only need this for security.ubuntu.com
- every time there is a rewrite of the Release file make Valid-Until valid for two more weeks
- if there was no security update for a week regenerate the valid-until in the release file (and resign of course) and make it valid for 2 weeks again
- when a distro is EOL the Release file does not have to be updated anymore. the user will see errors that $distro-security is no longer valid, but that is ok, because its a valid error and the user is no longer secure