WASHINGTON — Unable to meet tight deadlines in the new health care law, the Obama administration is delaying parts of a program intended to provide affordable health insurance to small businesses and their employees — a major selling point for the health care legislation. The law calls for a new insurance marketplace specifically for small businesses, starting next year. But in most states, employers will not be able to get what Congress intended: the option to provide workers with a choice of health plans. They will instead be limited to a single plan.