Handle missing user_creds in stack.delete

In some cases concurrent or interrupted and resumed stack delete
operations will result in missing user_creds records referenced in a
stack. This may be the cause of race conditions between deletes
resulting in unexpected behavior and spurious failures.

Change-Id: I76f3f8803a4f9171223783a9c6eea0fc20280010
Related-Bug: #1306029
Merge "Handle missing user_creds in stack.delete"