add check for existing cluster

[leahecole]

Description

Because we have added retries for service errors, if that retry happens, the fixture that creates the cluster will fail on an "AlreadyExists". This PR modifies to say that "AlreadyExists" is acceptable - we can utilize that cluster, we just need to still tear it down eventually (which we were already doing 😄 )

Fixes #9513

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.9 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[leahecole]

@nicain looks like we're seeing quota issues. Is this related to last week's change in region?