Enable the memory cgroup for the Docker plugin.

This will allow for the enforcement and tracking of memory limits and usage.
This successors line is good. Also put boot.ConfigureGrub and (until it gets consolidated properly) bootstrapvz.providers.gce.tasks.boot.ConfigureGrub) as predecessors, unless the relative order of these tasks doesn't matter, in which case ignore this comment. My initial reaction is that you can indeed ignore this safely, but it's worth thinking about.