By Martyn Herman RIO DE JANEIRO, Aug 17 (Reuters) - World road champion Peter Sagan will add some glamour to the Olympic cross-country race this weekend, having gone back to his mountain bike roots for a crack at Olympic gold. A former world junior champion in cross-country, Sagan has carved out a glittering career as a road racing professional, winning the Tour de France points category five years in a row and the rainbow jersey in Richmond, Virginia, last year. There was surprise when the Slovakian, 26, decided against competing in the Olympic road race and world cycling president Brian Cookson suggested he might have been kicking himself after Belgium's Greg van Avermaet won gold on the Copacabana. More...