OpenAPI Validation for PredictiveUnits limited to 5 levels

[ukclivecox]

As CRD OpenApi Validation does not allow recursive definitions at present the CRD has a fixed expanded definition of the children Predictive Units:

https://github.com/SeldonIO/seldon-core/blob/master/operator/config/crd/patches/graph_children.yaml

This needs to be expanded as a short term fix to more levels as users are hitting these levels of graph complexity.

A long term solution would be when CRD OpenAPI validation allows recursive definitions.
At present this seems to have not moved forward: kubernetes/kubernetes#54579