Add support for image namespace override

[burmanm]

What is missing?

In some cases, airgap installations require the namespace to be overridden as well for all images. Currently, imageConfig does not support this.

What we need is a feature that modifies for example k8ssandra/cass-management-api images to be internal/cass-management-api.

Why is this needed?

This is required by some airgap installations, which bundle all the images to a single namespace and do not retain the original one (so just changing registry will not work).

┆Issue is synchronized with this Jira Story by Unito ┆Reviewer: Alexander Dejanovski ┆Issue Number: CASS-8