Umbrella Issue: #1735

Some manifests (e.g., notebooks controller, katib) have multiple versions. Take
the example of notebook-controller:

There are the following versions of manifests:

• The ones living in the upstream repo, used in development and testing.
• The base and overlays folders, in kubeflow/manifests. These were
  optimized to work with kfctl's kustomization combination magic.
• The base_v3 folder, designed to eschew vars and be buildable with
  kustomize. It imports files from base.

Consolidate multiple manifests versions into one.