Pepsi stock has gone practically nowhere for five years, but this activist investor believes there's a clear path for turning the ship around.PepsiCo (PEP -0.28%) soared as much as 7.2% intraday on Sept. 2 in response to news that activist investor Elliott Investment Management took a $4 billion stake in the beverage and snack giant. With a market cap of around$200 billion, the stake gives Elliott roughly 2% ownership in the company.But since then, Pepsi's shares have tumbled, down to $141.71 as of market ...