[CWS] build functional tests with DNF tag

[paulcacheux]

What does this PR do?

This PR adds support for dnf backend in CWS functional tests.

Please note that the SLES tests are not completely working yet, because of repository issues.

Motivation

Fix SLES tests.

Possible Drawbacks / Trade-offs

None

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• The appropriate team/.. label has been applied, if known.
• Use the major_change label if your change either has a major impact on the code base, is impacting multiple teams or is changing important well-established internals of the Agent. This label will be use during QA to make sure each team pay extra attention to the changed behavior. For any customer facing change use a releasenote.
• A release note has been added or the changelog/no-changelog label has been applied.
• Changed code has automated tests for its functionality.
• Adequate QA/testing plan information is provided if the qa/skip-qa label is not applied.
• If applicable, docs team has been notified or an issue has been opened on the documentation repo.
• If applicable, the need-change/operator and need-change/helm labels have been applied.
• If applicable, the config template has been updated.