We're seeing this host need pruning more than once a month. Attempt to
get ahead of that a bit more by retiring backups for old systems. I
have also added a TODO to purge the retired systems that haven't been
purged yet. Purging removes the last backup retained after retirement so
should be done when we're really ready to do so.
But that also frees up a fair bit of space so we shouldn't not do it.
Change-Id: I072d70b6269a00b99986e465b31cc1b20b57ba51
5c60ee1bed