Bug fix for multi credential

Currently we don't fill the context map with workerIndex (renamed to iterationIndex), it is a bug introduced from PR #91, and we didn't have a test in ExecuteConcurrently() to make sure the index is populated so the bug slipped by the test.

Fixes:
- Populate index in ExecuteConcurrently(), renamed workerIndex to iterationIndex because the index is actually # of iteration
- Test to make sure ExecuteConcurrently() populates the context map with 'iterationIndex'