[Bug] Use DomainReadOnlyUser from the input provided in config

[himani2411]

Description of changes

Use DomainReadOnlyUser from the input provided in config

• Making the function case-sensitive
• making the function stricter as cn can be part of username

Current Logs show

[2025-05-24T01:14:22+00:00] INFO: Failed to retrieve the ReadOnlyUser from CN=ReadOnlyUser,OU=Users,ou=MICROSOFTAD,DC=microsoftad,DC=kgqvfhzxwz,DC=multiuser,dc=pcluster
--
[2025-05-24T01:14:22+00:00] INFO: Falling back to the default UserName ReadOnlyUser

this is a blocker if input is CN=ReadUser,...

Tests

Cluster-creation with AD
Logs

[2025-05-27T21:07:00+00:00] INFO: Token is CN=ReadOnlyUser  and condition true ##### Log Line I added for debugging but not part of PR
[2025-05-27T21:07:00+00:00] INFO: UserName for ReadOnly directory service user set to ReadOnlyUser

References

• Link to impacted open issues.
• Link to related PRs in other packages (i.e. cookbook, node).
• Link to documentation useful to understand the changes.

Checklist

• Make sure you are pointing to the right branch.
• If you're creating a patch for a branch other than develop add the branch name as prefix in the PR title (e.g. [release-3.6]).
• Check all commits' messages are clear, describing what and why vs how.
• Make sure to have added unit tests or integration tests to cover the new/modified code.
• Check if documentation is impacted by this change.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.50%. Comparing base (54b9986) to head (deb4057).
Report is 7 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2961   +/-   ##
========================================
  Coverage    75.50%   75.50%           
========================================
  Files           23       23           
  Lines         2356     2356           
========================================
  Hits          1779     1779           
  Misses         577      577           

Flag	Coverage Δ
unittests	75.50% <ø> (ø)

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