SERVER-14118 Added  ability to  detect if _id is missing when  passing an object to rs.add, if missing add it automatically  as if it was a string that had been passed.

Signed-off-by: Benety Goh <benety@mongodb.com>
SERVER-14118 omit _id from rs.add() configuration document in test
SERVER-14118 removed redundant cfg._id == undefined check from rs.add()