Hello
from this post it’s said that the goal is to 'adding cloud-init functionality to all distro images that can support it ', as such all distro are building a cloud variant but if cloud-init is not supported a ‘cloud’ labelled image is built but is identical to the default one. It’s confusing but it’s like that.
See this file, you’ll notice that a ‘variant’ is included in the jenkins axes but the build instruction don’t use the variant variable (see for example fedora version where the build instruction does use the variant variable at line 55)
Perhaps the problem comes from Debian 8 that does not inits correctly at the moment (systemd-networkd goes into an infinite loop), so of course cloud-init would not work.
Anyway, you could use distrobuilder yourself to try to build a debian 9 image, if there is no basic problem in Debian 9 you would get a cloud-init enabled image. In your place I’d try first to install cloud-init manually in a debian 9 container to see if it works.