Fix STS AssumeRole for third party delegation

For third party account delegation, you need to use the
ExternalId parameter, not UniqueClientId according to the
docs (and testing it out).

Adjust the paramter accordingly
Merge branch 'katzj-iam-assume-role-external-id' into develop

* katzj-iam-assume-role-external-id:
  Fix STS AssumeRole for third party delegation