Glenn Lowson/NP/Files Billionaire Michael DeGroote is accused of consorting with mobster Vito Rizzuto. A bitter dispute over a US$112-million investment in Caribbean casinos has placed Michael DeGroote, one of Canada’s wealthiest businessmen, at the centre of bizarre accusations of Mafia exploitation, death threats and fraud. Mr. DeGroote, 81 – hailed as a self-made billionaire who was…