docs: def change_password(self, usrname, oldpwd, newpwd, callback=None):
          '''
          Change password.
          '''
          params = {'usrName': usrname,
                    'oldPwd' : oldpwd,
                    'newPwd' : newpwd,
                   }
          return self.execute_command('changePassword',
                                      params, callback=callback)