release-23.2: roachprod: randomize default zone

[blathers-crl]

Backport 1/1 commits from #120714 on behalf of @renatolabs.

/cc @cockroachdb/release

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As a stopgap measure to reduce the chances of "zone exhausted" errors we see during roachtest runs¹, we randomize the default zone used when creating clusters with roachprod.

Informs: #114523

Release note: None

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Release justification: test only changes.

Footnotes

1. for an example, see https://github.com/cockroachdb/cockroach/issues/120621#issue-2191271023 ↩

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

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  issues or test-only changes.
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[cockroach-teamcity]

This change is 

[renatolabs]

TFTR!