Update proxy config if present

[dhiltgen]

If the system has a containerd-proxy managing the lifecycle
of the daemon, make sure the config is updated with the new image
on update.

Example manual test scenario:

root@daniel-testkit-B217D2-ubuntu-0:~# cat /etc/containerd-proxy/dockerd.json 
{
    "image": "docker.io/docker/engine-community",
    "imagePath": "/var/lib/docker-engine/engine.tar",
    "namespace":"docker",
    "args": [
        "--containerd", "/run/containerd/containerd.sock",
        "--default-runtime", "containerd",
        "--add-runtime", "containerd=runc"
    ],
    "scope": "ce"
}
root@daniel-testkit-B217D2-ubuntu-0:~# docker version --format '{{.Server.Version}}'
18.09.0-ce-tp4
root@daniel-testkit-B217D2-ubuntu-0:~# ./docker engine update --version 0.0.0-20180828050111-3bb99a8
0.0.0-201808: resolved 
waiting for engine to be responsive... engine is online.
Success!  The docker engine is now running.
root@daniel-testkit-B217D2-ubuntu-0:~# cat /etc/containerd-proxy/dockerd.json
{"namespace":"docker","image":"docker.io/docker/engine-community:0.0.0-20180828050111-3bb99a8","imagePath":"","args":["--containerd","/run/containerd/containerd.sock","--default-runtime","containerd","--add-runtime","containerd=runc"],"scope":"ce"}
root@daniel-testkit-B217D2-ubuntu-0:~# docker version --format '{{.Server.Version}}'
0.0.0-20180828050111-3bb99a8

[crosbymichael]

LGTM

[andrewhsu]

@dhiltgen needs port to master branch?

[dhiltgen]

@dhiltgen needs port to master branch?

Yes, I'll get a cherry-pick going shortly.

[selansen]

don't we need to check "enc != nil" before we call enc.Encode?

[dhiltgen]

it can never be nil

[selansen]

LGTM