Cherry-pick #23496 to 7.x: Agent fetching DBus service PID fix

[michalpristas]

Cherry-pick of PR #23496 to 7.x branch. Original message:

What does this PR do?

For some reason debian9 is not happy when quering service without .service suffix just by a service name.
Adding that debian does not complain anymore

Why is it important?

This prevents us from monitoring PID and checking agent crashes. If agent crashes constantly status checker will know and rollback instead of this crash checker, so i would not say it's that critical.

Fixes: #23488

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)

[elasticmachine]

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Build stats

• Build Cause: Pull request #23525 updated

• • Start Time: 2021-01-17T13:32:13.357+0000

• Duration: 20 min 12 sec

• Commit: b7f427c

Test stats 🧪

Test	Results
Failed	0
Passed	1414
Skipped	6
Total	1420

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Test stats 🧪

Test	Results
Failed	0
Passed	1414
Skipped	6
Total	1420

[blakerouse]

Backport looks good.