Add unit test for image management code.

[Random-Liu]

The image management code is merged #21.

However, we don't have unit test for it, because:

1. The fake containerd service is not ready.
2. Containerd is still changing their image side api.

We should:

1. Add unit test as much as we can for now, e.g. current ListImages and ImageStatus purely based on metadata store, it's testable.
2. Add unit test with fake containerd service when containerd stabilizes their api and We need fake containerd services. #20 is finished.

[Random-Liu]

We could add better unit test, after switching to new containerd client.

[Random-Liu]

This work depends on the new containerd client switch, punt to v0.2.0.

[Random-Liu]

This is relatively lower priority, and I don't think we have time to get to this this release.

[Random-Liu]

I think we've got enough test coverage. Let's close this one.