landscape-client shouldn't remove UI packages in released ubuntu series

[Ursula Junque]

With the landscape-client UI drop, landscape-client package was updated to remove obsolete ui packages.
This represents an issue for released ubuntu series such as wily and trusty, as released packages can't be removed after a series is released (landscape-client-ui-install is seeded in series older than xenial). For these series the packages shouldn't be removed, but instead replaced by empty ones (or something equivalent). The way the package is right now won't be accepted as a SRU for this reason.

The goal is to have one same source building packages for all releases, we need a conditional package build for xenial and < xenial, with different options.