Cherry-pick #24466 to 7.x: Logging to file disabled on enroll

[michalpristas]

Cherry-pick of PR #24466 to 7.x branch. Original message:

What does this PR do?

The problem here is that enroll runs simultaneously with running service and both try to log into same file.
Although for enroll there's no need to do that as it is invoked either manually (with std output) or as executed from install command with std redirecting to a caller.

This PR makes sure to create internal logging file elastic-agent-json.log only in case it is really needed.

Why is it important?

Fixes: #24173

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.

[elasticmachine]

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

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[blakerouse]

Backport looks good.

[elasticmachine]

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Build stats

• Build Cause: Pull request #24473 opened

• Start Time: 2021-03-11T08:07:31.620+0000

• Duration: 54 min 28 sec

• Commit: 3949300

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Test	Results
Failed	0
Passed	45985
Skipped	4982
Total	50967

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Test stats 🧪

Test	Results
Failed	0
Passed	45985
Skipped	4982
Total	50967