BRISTOL, Tenn. — There’s an expectation from fans that a ticket to Bristol Motor Speedway will get them a seat to NASCAR’s version of the Roman Colosseum. They got one of those throwback, rock ‘em, sock ‘em races last August, when changes to the track surface forced drivers to get aggressive again and caused tempers to flare. Now, a month into a new Sprint Cup season, NASCAR could use another race like that. Sunday’s race will be the fourth for the new Gen-6 car, and the first this season on a short track. It could be the spark NASCAR needs …