Cloud: Unnecessary wrapping lambda parameters in `args` attribute

[cheungpat]

Currently parameters for lambda are obtained like this:

skygearCloud.op('foo', function (param, options) {
  const {
    context
  } = options;
  console.log(param['args']); // the parameters passed by SDKs
});

Since the parameters is always wrapped in args and developers are not expected to get anything outside the args dictionary, wrapping the lambda parameters in args are unnecessary.

This is a breaking change.

[carmenlau]

Need update the guides too when update