This reverts commit c81bb20f5b.
After re-reading the documentation: "The Recommends field should list packages that would be found together with this one in all but unusual installations."
Thus, "Recommends" is an acceptable place for this dep, and anyone disabling that gets to keep the pieces.
The main crux of why this needs to be reverted is because it breaks Debian completely because "lxc" and "cgroup-bin" can't be installed concurrently.
Since cgroup-bin is only "recommended" by the lxc package on Ubuntu, but is necessary for having the proper cgroups mounted for Docker to function, this makes some sense for us to add separately.
Fixes#2990
