* Bogey-free final round for champion Porteous * Trio seal Open berths at Royal Troon (Adds details) JOHANNESBURG, Jan 17 (Reuters) - South African Haydn Porteous held his nerve for a bogey-free round of 69 to claim a maiden European Tour victory at the Joburg Open on Sunday. Joint overnight leader Porteous made just five bogeys all week and hit every green in regulation at the Royal Johannesburg and Kensington Golf Club during his final round, while the chasing pack behind him struggled for the same consistency. The 21-year-old finished the tournament on 18-under-par, two shots ahead of compatriot Zandersaf Lombard (71). More...