Upgrading can have no specific knowledge about additional
tasks besides upgrading the configuration, as upgrades need
to be able to go to any future version (within the same major
version). Downgrading, however, is version specific and always
goes to the previous minor version. Therefore, the downgrade
procedure can do arbitrary specific tasks prior to switching
to the new binaries. Add a hook for this.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>