Allow easy override of the registry and pull secrets

[adejanovski]

We currently rely on the imageConfig to set up overrides on the registry and pull secrets which are used to create the image coordinates of the pods cass-operator generates.

We don't have a similar system in k8ssandra-operator, but could get this with low effort.
The Reaper and Medusa images use the images package with the Image struct that separates the different fields that are part of image coordinates, and the pull secrets.

We can add env variables to create overrides for the default registry and the pull secret name which would allow to globally point to a specific registry.

Definition of Done

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1. default registry and pull secrets can be overridden in the helm chart