BOGOTA (Reuters) - Gustavo Petro, a former insurgent turned rabble-rousing mayor, is fighting to become Colombia's first leftist president, turning his humble beginnings and revolutionary past into a rallying cry for millions of the nation's youth and poor. The 58-year-old economist is about 20 points behind right-wing candidate Ivan Duque to replace President Juan Manuel Santos, but his presence in the second round has worried many in the traditionally conservative nation, where the right has kept a vice-like grip on power. Proposals from Petro to change the nation's economic model by piling taxes on unproductive landowners and abandoning oil and...