app-emulation/containerd: Enable the CRI plugin

Kubernetes uses containerd through the cri plugin which currently is
disabled due to it listening on a TCP port. Now the plugin is not
listening on a TCP port anymore but uses the same socket as gRPC.
We have documented how to enable it in
https://kinvolk.io/docs/flatcar-container-linux/latest/container-runtimes/switching-from-docker-to-containerd-for-kubernetes/
but it should work by default.

Fixes flatcar/Flatcar#283

app-emulation/containerd/files/config.toml

@@ -6,8 +6,7 @@ state = "/run/docker/libcontainerd/containerd"
 subreaper = true
 # set containerd's OOM score
 oom_score = -999
-# CRI plugin listens on a TCP port by default
-disabled_plugins = ["cri"]
+disabled_plugins = []
 
 # grpc configuration
 [grpc]