using fstrim to save some space in cloud images

[Peter Robinson]

> Hey all,
>
> Have we considered running fstrim against our cloud image filesystems
> before we package it up? I wrote a small script to do it (inside a
> container) at [1]. Looks like we can save ~28M:

It would need to be incorporated into the image build process because
it would affect checksums etc. I believe at least some of the tools we
use already support zeroing out free space and similar, maybe we just
need to be sure the right options are being passed through.

Peter