Support for Kubernetes List documents

[squaremo]

We can't update policy or image refs inside List documents, and I suspect we can't sync them properly either.

If you are pointing flux at an existing git repo with your yamls, this is one of the common causes for it to inexplicably break

[squaremo]

OK I've tried it. For comparison, it's possible to

$ kubectl apply list.yaml

and it will create the resources therein listed.

If I add list.yaml to the repo I'm syncing, it will get applied (and the resources created) -- but flux does not recognise the individual resources, it thinks it's syncing a single resource called list/ (i.e., with no name). This will probably go wrong if there's more than one list documents.

[squaremo]

When we fix this, we should also remove its mention as a limitation in #820 .

[squaremo]

List documents are supported as of #976