Add `genome disk allocation purge` CLI

It requires a `reason` and a list of `Genome::Disk::Allocation` objects.

I'm just adding this for convenience.
If you care, `*_message` has built in `sprintf`, so you can do something like `$self->status_message("Skipping allocation %s, status already purged", $allocation->id`. Same for the other status messages in the command. Really just personal preference either way though.