DELTA, British Columbia — Mat Beren and his friends used to drive by the vast greenhouses of southern British Columbia and joke about how much weed they could grow there.Years later, it’s no joke. The tomato and pepper plants that once filled some of those greenhouses have been replaced with a new cash crop: marijuana. Beren and other formerly illicit growers are helping cultivate it. The buyers no longer are unlawful dealers or dubious medical dispensaries; it’s the Canadian government.On Oct. 17, Canada becomes the …