pull out env vars from daemonset and document them

[gsantoro]

What does this PR do?

Document environment variables in .env files instead of leaving comments in daemonset. it also adds extra documentation

Why is it important?

It makes clear where to define env variables

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Relates Use Kustomize to build elastic-agent manifests for both managed and standalone more #2104

[mergify]

This pull request does not have a backport label. Could you fix it @gsantoro? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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• Start Time: 2023-01-24T10:04:09.065+0000

• Duration: 18 min 20 sec

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Test	Results
Failed	0
Passed	4833
Skipped	13
Total	4846

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[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	98.305% (58/59)	👍
Files	69.268% (142/205)	👍
Classes	69.231% (270/390)	👍
Methods	54.047% (828/1532)	👍
Lines	39.31% (9037/22989)	👎 -0.03
Conditionals	100.0% (0/0)	💚

[ChrsMark]

Thanks for for the PR @gsantoro !

For my understanding: so based on these (and previous) changes we only need to define any ENV vars inside the .env files instead of having them in the k8s manifests, right?

Changes look good to me. Do we have an issue or dev doc page that explain what is the purpose of this wider change, what is missing and what are the steps forward? Sorry if I have missed it :).

[gsantoro]

@ChrsMark a bit more context was linked at #1668