Fix num of DRep credentials generated for transient parameter

[palas]

Changelog

- description: |
    Fixed number of `DRep` credentials generated by `create-testnet-data`.
  type:
  - bugfix

Context

The number of DRep credentials generated by create-testnet-data when the --transient-drep-keys parameter was used was ignored, and the number used was that for stake delegators. This PR fixes this. This was discovered in this PR: #943

How to trust this PR

You can see that this change makes the branch for transient consistent with the other branch.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff

[carbolymer]

Hydra darwin got stuck, force merging.