By Carolyn Cohn, Pamela Barbaglia and Valentina Za LONDON (Reuters) -A rebel investor in Generali could mount a legal challenge if his bid to appoint new top executives at Italy’s largest insurer loses by a narrow margin in a shareholder vote this month, the candidate to the chairman role said. In an interview with Reuters, former Goldman Sachs banker Claudio Costamagna said Generali investor Francesco Gaetano Caltagirone could challenge in court a victory by the opposite side at the April 29 AGM called to elect a new Generali board if the margin is less than 6%. He also said the Caltagirone c…