A trucker from California will spend at least 20-months in state prison for hauling a load of cocaine into Montgomery County.   42-year-old Crecenciano Rodriguez-Estrada was arrested more than a year ago at a WalMart on Route 29 in Upper Hanover Township.   The cocaine was concealed in a hidden compartment on the roof of the tractor trailer.   Investigators estimate its street value is more than $500,000.