feat: Make OPENSEARCH_HOME and OPENSEARCH_PATH_CONF overridable

[siegfriedweber]

Description

Make OPENSEARCH_HOME and OPENSEARCH_PATH_CONF overridable

In the early development phase of the operator, it is extremly useful, to be able to use the official product image. On the one hand, different behaviors can be tested, on the other hand, other product versions like OpenSearch 2.x can be tested where no Stackable image exists.

Part of #2

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

• Changes are OpenShift compatible
• Helm chart can be installed and deployed operator works
• Integration tests passed (for non trivial changes)
• Changes need to be "offline" compatible
• Links to generated (nightly) docs added

Reviewer

• Code contains useful comments
• Code contains useful logging statements
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

• Feature Tracker has been updated
• Proper release label has been added
• Links to generated (nightly) docs added
• Release note snippet added
• Add type/deprecation label & add to the deprecation schedule
• Add type/experimental label & add to the experimental features tracker

[adwk67]

I haven't have run the tests but this is LGTM assuming you have done so!
Update: local tests all good

[adwk67]

CHANGELOG is missing

[adwk67]

Thanks!

[siegfriedweber]

Documentation

https://docs.stackable.tech/home/nightly/opensearch/usage-guide/configuration-environment-overrides/#_environment_variables