Fix pod template envvars

[adwk67]

Description

See #371.
The envOverrides entries are added to the config-map for the kubernetesExecutor pod template by iterating over a HashMap: this leads to a unpredictable order and unnecessary restarts of airflow components. The HashMap is converted to a BTreeMap to avoid this happening.

🟢 https://ci.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/airflow-operator-it-custom/121/

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1. CRD documentation for all fields, following the style guide.
2. Helm chart can be installed and deployed operator works
3. Integration tests passed (for non trivial changes) https://ci.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/airflow-operator-it-custom/121/

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1. Code contains useful comments
2. (Integration-)Test cases added
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[lfrancke]

I'm curious: Can you explain why the unpredictable ordering causes restarts? I'm not sure I understand yet.

Also: Would this affect other operators as well?

[lfrancke]

Also: Good find!

[adwk67]

Before this change the configMap gets written up to three times (same UID but different resource versions) - to be honest, I'm not sure why - and the ordering was not consistent. But it's one of the few cases where we write env-vars to the config map as it is for the executor template. As airflow (and superset) both have restarts annotated this causes the phenomenon: we should probably change this for common_config.env-overrides so that any callout gets an ordered map. Props go to @nightkr, who suggested the HashMap could be the cause!

[NickLarsenNZ]

Nice, and I appreciate the PR description :)

[soenkeliebau]

awesome find Andrew, thanks!