Code review comment for lp://staging/~jmarsden/ubiquity/lp775124

I don't know why I didn't make this comment earlier, but I do not believe we should be special casing Ubuntu flavors here at all.

For what it's worth, the fudge factor is there not as a workaround, but because we do not want users creating an Ubuntu installation that is only just big enough to contain the system files. We should ensure they have enough room to be able to use the operating system, including storing documents, music, and downloads, for the life of the installation.