Some of the largest health insurers in Massachusetts say they will cover visits to the retail health clinics expected to open in CVS and Walgreens drugstores later this year. Harvard Pilgrim Health Care and Tufts Health Plan have signed contracts with CVS Caremark, which operates more than 6,000 pharmacies nationwide. The chain plans to open as many as 28 MinuteClinics in its Massachusetts stores this year and 100 statewide within five years. CVS also is negotiating coverage for clinic services with Blue Cross and Blue Shield of Massachusetts. Together, the insurers have nearly 5 million members.