Plot: The Ravenswood Billboard Factory in Chicago, which now serves as a home for one of the nation's premier collections of vintage sports cars, provided a colorful venue for a secret concert by multiplatinum country singer Dierks Bentley on a cool April night. Built in 1920, the former paper warehouse and billboard photography studio's high vaulted ceilings gave plenty of room for the rousing sound of Bentley's free 'n easy brand of country