Cherry-pick #118 to 7.x: Set agent active to false on unenroll

[blakerouse]

Cherry-pick of PR #118 to 7.x branch. Original message:

What does this PR do?

Fixes issue where .fleet-agents active field is not set to false on un-enrollment. Without setting that field to false Kibana will still show the agent after successful un-enrollment.

Why is it important?

So unenroll action works correctly.

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.

[aleksmaus]

Still approve

[elasticmachine]

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Build stats

• Build Cause: Pull request #120 opened

• Start Time: 2021-03-03T21:16:09.378+0000

• Duration: 10 min 58 sec

• Commit: 0b6dc96

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Test	Results
Failed	0
Passed	50
Skipped	0
Total	50

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