In Kayole, a slum on the outskirts of Nairobi, Kenya, Joab Omondi has been selling water from a cart since the government connected his neighborhood to the local water system. Two years ago, water was a scarce commodity here. Residents used to have to hunt around to buy a 5-gallon can of water for 30 shillings—about 30 cents. Now the situation has changed since the World Bank and the Nairobi City Water and Sewerage Co.…