GET /credentials does not return credential_id

[SmithSamuelM]

This issue is a result of figuring out the issue #163

The problem is that GET /credentials does not return the credential_id needed to use GET /credentials/{id}

The only place to find credential_id is in the associated exchange record but finding the appropriate record may be problematic. But once found the record has three credential ID related fields. These are

cred_def_id
credential_exchange_id
credential_id

It would be great to document in the Swagger interface or other docs that there are indeed Three different ids that the ACA-PY uses and it would simplify use of the API if GET /credentials returned the right one needed for the GET /credentials/{id}