Use a data source instead of a resource to look up workspaces in the TF Cloud credentials module

[mcgirr]

• [x] Update the changelog
• [x] Make sure that modules and files are documented. This can be done inside the module and files.
• [x] Make sure that new modules directories contain a basic README.md file.
• [ ] Make sure that the module is added to tests/main.tf
• [ ] Make sure that the linting passes on CI.
• [ ] Make sure that there is an up to date example for your code:
  • For new modules this would entail example code for how to use the module or some explanation in the module readme.
  • For new examples please provide a README explaining how to run the example. It's also ideal to provide a basic makefile to use the example as well.
• [ ] Make sure that there is a manual CI trigger that can test the deployment.

[mcgirr]

This should be ready to go now.

[mcgirr]

We should keep the branch associated with the PR until there is a new release.

[mcgirr]

@ketzacoatl I will resolve the merge conflcts on this but it will be important to get this into a new release. The one that is currently on master should not be used because it uses a resource instead of a data source in data.tf (which is why so far we've been using this branch's ref in our module sources where this is used).