Cherry-pick #22815 to 7.x: Push log level downstream

[michalpristas]

Cherry-pick of PR #22815 to 7.x branch. Original message:

What does this PR do?

This PR injects log level settings into fleet section of config as discussed in linked issue.
resulting config will look like this

fleet:
  agent:
    id: {agent_id}
    logging:
      level: warning
  api:
    access_api_key: {agent_api_key}
    kibana:
      host: {host_uri}:443
      hosts:
      - {host_uri}
      protocol: https
      timeout: 5m0s
  host:
    id: {host_id}
inputs:
- artifact_manifest:
    artifacts:
      endpoint-exceptionlist-macos-v1:
...so on

Why is it important?

#20756

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.next.asciidoc or CHANGELOG-developer.next.asciidoc.

cc @ph @ferullo

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Pinging @elastic/ingest-management (Team:Ingest Management)

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[elasticmachine]

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• Build Cause: Pull request #22985 opened

• Start Time: 2020-12-08T13:11:07.179+0000

• Duration: 18 min 14 sec

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Passed	1384
Skipped	6
Total	1390

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Test stats 🧪

Test	Results
Failed	0
Passed	1384
Skipped	6
Total	1390

[ferullo]

Nice thanks! Endpoint has already merged a change to start reading from this location.

[blakerouse]

Backport looks good.