(Reuters)   Oil prices tumbled on Monday after a meeting by major producers in Qatar collapsed without an agreement to freeze output, leaving the credibility of the OPEC producer cartel in tatters and the world awash with unwanted fuel. Geopolitical tensions between Saudi Arabia and Iran were blamed for the failure, which revived industry fears that major government-controlled producers will increase their battle for market share by offering ever-steeper discounts. “OPEC’s credibility to coordinate output is […]