Add restriction for values of execute_mode

[tobegit3hub]

Now users can set any string value to execute_mode which is invalid and should be prevented when setting.

[vagetablechicken]

I've thought about it. It's better to define how to use it first(we may need convert string to enum), then add the assign restriction(string to enum too). So I leave it.

[vagetablechicken]

Not only the value, we'll add restriction for variable keys. (We don't support to use user-defined vars, so it's ok to restrict the variable names)