By Heide Brandes OKLAHOMA CITY (Reuters) - Music stars from across the nation joined forces in Oklahoma on Wednesday in a benefit concert for victims of the deadly tornado that struck the state last week. Tickets for the "Healing in the Heartland" concert, held at Oklahoma City's roughly 20,000-capacity Chesapeake Arena and broadcast on NBC television, sold out in minutes when they went on sale on Saturday. It closed with a duet between Oklahoma-born country singer Blake Shelton, who is a judge on NBC singing competition "The Voice," and Grammy winning R&B star Usher. ...