BOSTON — Massachusetts has a cannabis conundrum.State law requires every county to have a dispensary for medical marijuana, but federal regulations forbid the drug from being transported across U.S. waters. That leaves the tourist havens of Martha’s Vineyard and Nantucket potless unless someone gets a license to grow it legally on the islands.Despite their small size, both islands are their own counties.State officials suggest the federal restrictions could add to the challenge of opening medical marijuana …