SMITHFIELD — The Johnston County hospital system has become the latest community care provider to align itself with a much larger partner, selecting UNC Health Care to help it expand services and lower operating costs. The decision by Johnston Health, which operates a hospital in Smithfield and an outpatient clinic in Clayton, represents a victory for UNC over its rival WakeMed, which was one of two other possible partners that Johnston had been considering. It is also another example of the type of hospital consolidation that some believe is driving up the cost of health care for patients and employers.