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

Tested on a local system-image server. I checked both the case of setting a custom phased percentage value and leaving the default value (which currently is 100%). We can do it that way as set_phased_percentage doesn't write down the value if it's 100 (since then there's simply no entry for phased percentage)