Code review comment for lp://staging/~sil2100/ubuntu-system-image/server-copy-image-phased

This looks good to me. I thought about whether the percentage should be an optional argument to create_image() itself, but looking at it I don't see think that's advantageous because it implies that you could have two different update paths (full update vs. delta) that have different phased percentages andwe specifically don't want to allow that. So keeping the existing set_phased_percentage() api avoids any accidents there.