* Properly return ImageNotFoundError
* Support inject "Images" or "ImageInspects" and keep both in sync.
* Remove the FakeDockerPuller and let FakeDockerClient subsumes its
functinality. This reduces the overhead to maintain both objects.
* Various small fixes and refactoring of the testing utils.
We have observed that, after failing to create a container due to "device or
resource busy", docker may end up having inconsistent internal state. One
symptom is that docker will not report the existence of the "failed to create"
container, but if kubelet tries to create a new container with the same name,
docker will error out with a naming conflict message.
To work around this, this commit parses the creation error message and if there
is a naming conflict, it would attempt to remove the existing container.
