Support schema 1 manifest.

[Random-Liu]

Currently, containerd only supports schema 2 manifest.

For cri-containerd, not supporting schema 1 means:

1. Users who are still using schema 1 image could not use cri-containerd integration.
2. Some system container images are still using schema 1 manifest, e.g. pause image.

Regarding to 1), it is fine for our alpha release (this quarter), but it should be fixed before beta (probably the end of the year).

For 2), we need to rebuild our schema 1 system container image.

Ref containerd/containerd#851.

[Random-Liu]

This become the bigest blocker for running node e2e test.
Many images used by node e2e test are using schema 1 image.

Except that, other 2 high priority work for testing:

1. Implement container log;
2. ExecSync.

[crosbymichael]

What is ExecSync?

[Random-Liu]

@crosbymichael It's defined in CRI, https://github.com/kubernetes/kubernetes/blob/master/pkg/kubelet/apis/cri/v1alpha1/api.proto#L66.

Basically exec a command in container and return result synchronously. We use it a lot in our integration test.

[Random-Liu]

@crosbymichael I might have mislead your guys, the other 2 items I mentioned are purely work on our side.

[Random-Liu]

This is finished in #81.

Close this one.