Use workload's format for images

[squaremo]

When replacing images in a workload, it'd be good to keep the format that the workload itself uses, and just replace the tag (or digest, if that's supported). E.g., if the workload uses

image: helloworld:3.8

and the policy has

status:
  latestImage: index.docker.io/library/helloworld:3.9

then the update should be to

image: helloworld:3.9

[squaremo]

I need to decide whether this is still relevant for the setters mechanism #23 (as opposed to the simple update mechanism, which was removed in #27).

[squaremo]

This may be more to do with how the image reflector controller exposes the latest image -- the setters just uses the value as it is given in the ImagePolicy status.