Fast growth, a cheap valuation, and an unchanged royalty rate make Arcos Dorados stock a bargain.Shares of McDonald's (MCD -0.10%) South America, Central America, and Caribbean franchisee Arcos Dorados (ARCO 12.33%) leapt 16% higher through 9:45 a.m. ET Tuesday. The catalyst was what seems to be a rather run-of-the-mill announcement: Arcos Dorados has exercised its option to renew its master franchise agreement (MFA) with the restaurant chain.It's the specifics of the renewal, though, that are driving Arcos ...