* Tour starts with time trial in Utrecht * Boom rides for defending Tour champion Nibali's Astana team (Adds Astana statement, background) By Julien Pretot UTRECHT, Netherlands, July 4 (Reuters) - Dutchman Lars Boom will start Saturday's first stage of the Tour de France despite showing low levels of cortisol in a test on the eve of the race, his Astana team manager said. "Boom will take the start," Alexander Vinokourov told reporters. Astana, the team of defending Tour champion Vincenzo Nibali, are members of the MPCC (Movement for Credible Cycling) that applies stricter rules than the International Cycling Union (UCI) and the World Anti-Doping Agency (WADA). More...