Code review comment for ~seb128/casper:desktop-canary-tweaks

Thanks for the review. It sounded lika a +1 with suggestion for futur improvements so I merged that one for now, which should give us a working canary build without ppa and no regressions on days where there is a casper upload to Ubuntu which we didn't rebase yet.

I've no strong opinion on what's the right place to do the customization, we have a bunch of similar hacks in casper today which is why I added there but we can move things later if wanted

And on the 'check that the image is a canary one' we will check if there is a better way but that should go away anyway once canary becomes the new desktop installer