Refactor Kube module

[jml]

When coming back to this code, there were some design choices that I didn't understand. This PR makes things more clear to me-from-the-present.

Specifically,

• Rather than have a special kubeObjectFromValue (why?!), just provide a standard FromJSON instance for KubeObject
• Rather than preserve the AST (i.e. Value) so we can later extract the images, just extract the images when we parse the KubeObject in the first place, which spares us having to keep the Value around
• Rather than a custom image name parsing function, make Image instances of FromJSON and ToJSON
• Delete unused functions and move ones that are used only once into where clauses

Depends on #30

[jml]

Since posting this, I found a bug, which c51e859 fixes.