re-include executor/driver cores in spark-config

[adwk67]

Description

re-include executor/driver cores in spark-config.
Fixes #300.

CI: 🟢 https://ci.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/spark-k8s-operator-it-custom/113/

The resources test has been adapted to confirm that SPARK_EXECUTOR_CORES has been correctly set in the executor pods.

Definition of Done Checklist

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1. Changes are OpenShift compatible
2. Helm chart can be installed and deployed operator works
3. Integration tests passed (for non trivial changes)
4. 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. Feature Tracker has been updated
2. Proper release label has been added

[razvan]

lgtm