Cherry-pick #21473 to 7.x: Prompt only when agent is already enrolled

[michalpristas]

Cherry-pick of PR #21473 to 7.x branch. Original message:

What does this PR do?

Currently when you install Elastic Agent for the first time and then issue the enroll command it prompts the user "This will replace your current settings. Do you want to continue? [Y/n]" However, there are no current settings so it should not show this prompt. It should only be shown when overwriting existing settings.

Why is it important?

Fixes #20068

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)

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[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #21506 opened]

• Start Time: 2020-10-05T07:43:35.588+0000

• Duration: 35 min 3 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1384
Skipped	4
Total	1388

[blakerouse]

Backport looks good.