Fix value explanation in get_creds_by_roles

On I78a4884e980ef7d0103639cb3792a54c69fb7761 review, there was some
mis-updated explanation but we skipped to update it because the patch
took long time for review and that was not critical.
This patch does cleanup for it.

Change-Id: Icb0d4eea1a9c128abfea7f5c16e8478103ec1c49
1 file changed