Kubernetes Container Image Discovery

[Tehsmash]

Following design #527 we should implement container image discovery as part of the kubernetes provider added by #561.

The provider should list all the nodes in the cluster, and from the node's status field collect the list of containers which are currently stored in the node's container runtime. A ContainerImage asset should be returned for each discovered container.

We should use the reference/name with registry hash as the ID for the ContainerImage asset.
We should use the container image size as the ContainerImage asset size.
We should use the Node's architecture and OS as the ContainerImage architecture and os.