Disable updates for ssapshot.debian.org

Disable updates for ssapshot.debian.org

security updates are a separate apt source.

The point of using snapshot is to avoid pulling in uncontrolled
updates, so we need to disable security updates.

The non-security SUITE-updates are disabled by this too.  But
everything is on fire and I don't want another iteration while I
figure out the proper syntax for disabling only the security updates.

Signed-off-by: Ian Jackson <iwj@xenproject.org>