service discovery: Add missing cgroup mount (#1610)

[tbavelier]

What does this PR do?

Cherry-pick #1610

Motivation

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Additional Notes

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Minimum Agent Versions

Are there minimum versions of the Datadog Agent and/or Cluster Agent required?

• Agent: vX.Y.Z
• Cluster Agent: vX.Y.Z

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Checklist

• PR has at least one valid label: bug, enhancement, refactoring, documentation, tooling, and/or dependencies
• PR has a milestone or the qa/skip-qa label

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.76%. Comparing base (1590012) to head (5bbfb2f).

Additional details and impacted files

@@            Coverage Diff             @@
##            v1.12    #1617      +/-   ##
==========================================
+ Coverage   48.75%   48.76%   +0.01%     
==========================================
  Files         235      235              
  Lines       21120    21125       +5     
==========================================
+ Hits        10296    10301       +5     
  Misses      10287    10287              
  Partials      537      537              

Flag	Coverage Δ
unittests	48.76% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...r/datadogagent/feature/servicediscovery/feature.go	84.61% <100.00%> (+1.28%)	⬆️

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