fix: Don't default roleGroup replicas to zero when not specified

[sbernauer]

Description

Fixes #398
Part of stackabletech/issues#435

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1. Changes are OpenShift compatible
2. CRD changes approved
3. Helm chart can be installed and deployed operator works
4. Integration tests passed (for non trivial changes)
5. Changes need to be "offline" compatible

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1. Code contains useful comments
2. (Integration-)Test cases added
3. Documentation added or updated
4. Changelog updated
5. Cargo.toml only contains references to git tags (not specific commits or branches)

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1. Proper release label has been added

[fhennig]

LGTM, but what about an integration test for this? Might be a bit trivial but still 😬

Also, we should document the behaviour if the replicas are omitted.

[sbernauer]

You might be interested in stackabletech/documentation#461 :)
Regarding the integration test I see why we want it, but I think it this should be a unit test but well... we have problems with mocking k8s

[maltesander]

LGTM!