Specify fetch_workers config option to omnibus

What does this MR do?

So that we can control how many fetch attempts are made. Requires omnibus!37 (merged) to be merged.

Related issues

#6614

Checklist

See Definition of done.

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Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
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Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
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• Tests added
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