Comment 3 for bug 1598212

Likely need to keep both UBUNTU_CODENAME and VERSION_CODENAME in xenial forever, to avoid having to add a (very) icky "Breaks: snapd (<< fixed-version)" to base-files when we remove it.

Though, in the interest of correctness, I suppose that Breaks would also be necessary in yakkety, but maybe we can get away with handwaving past it, if it doesn't critically break snapd on upgrade.