code: def set_preshared_key(self, new_key):
          
          if self.data.get('preshared_key'):
              self.update(preshared_key=new_key)
docs: Set the preshared key for this VPN. A pre-shared key is only
          present when the tunnel type is 'VPN' or the encryption mode
          is 'transport'.
          
          :return: None