Configuration::compute_* is not invoked if RoleGroup object doesn't define `config` field

[nightkr]

This means that the following object works:

apiVersion: imaginary.stackable.tech/v1alpha1
kind: ImaginaryCluster
metadata:
  name: imaginary-cluster
spec:
  role:
    roleGroup:
      config: {}

But the following might be missing crucial default values:

apiVersion: imaginary.stackable.tech/v1alpha1
kind: ImaginaryCluster
metadata:
  name: imaginary-cluster
spec:
  role:
    roleGroup: {}