Use autopkgtest image based on stable

Recently, !498 (merged) fixed an issue on the autopkgtest job. The issue it fixes was because lxc-templates was deprecated in favor of distrobuilder, and the lxc-templates package was missing in the autopkgtest image. But the motivation for this issue is, as Terceiro stated in the related mail thread: "ci.debian.net uses lxc from stable". Since we aim at mimic ci.d.n, as much as possible, we should build the autopkgtest job on top of stable.

Any objection?