Redesign the image and kernel ociClaim

[luxas]

As per #298; we're gonna simplify

spec:
  image:
    ociClaim:
      ref: weaveworks/ignite-ubuntu:latest
      type: Docker

to

spec:
  image:
    # Any image that is following the OCI specification is valid here.
    ociRef: weaveworks/ignite-ubuntu:latest
    # Reserve the possibility to add fields for other details

(and the same for kernel)

[twelho]

After some additional thought, we're going to further shorten ociRef to just oci, it's enough to signal the resource should be retrieved from the OCI implementation. So the target design looks like this:

spec:
  image:
    # Any image that is following the OCI specification is valid here.
    oci: weaveworks/ignite-ubuntu:latest
    # Reserve the possibility to add fields for other details