code: def repl_member_add(self, params):
          
          repl_config = self.config
          member_id = max([member['_id'] for member in repl_config['members']]) + 1
          member_config = self.member_create(params, member_id)
          repl_config['members'].append(member_config)
          if not self.repl_update(repl_config):
              self.member_del(member_id, reconfig=True)
              raise ReplicaSetError("Could not add member to ReplicaSet.")
          return member_id
docs: create new mongod instances and add it to the replica set.
          Args:
              params - mongod params
          return True if operation success otherwise False