Make stemcell version used in tests configurable

[weili-broadcom]

Description

Please include a summary of the change.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Testing performed?

• Unit tests
• Integration tests
• Acceptance tests

Checklist:

• This PR is being made against the main branch, or relevant version branch
• I have made corresponding changes to the documentation
• I have added testing for my changes

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[linux-foundation-easycla]

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• ✅ login: weili-broadcom / name: Wei Li (1e6dc94)

[weili-broadcom]

Found a workaround in the pipeline.