ChoiceParameter should raise an error when passed invalid choice

[jotinha]

Description

Validates ChoiceParameter when initializing a task by moving the logic from .parse to .normalize. Currently this validation only runs when calling from the command line, when the method .parse is called. Moving it to .normalize ensure it always runs

Motivation and Context

I was having an issue where I was requiring a task with an invalid ChoiceParameter but no ValueError was raising at task init time.

class Foo(luigi.Task):
     args = luigi.ChoiceParameter(var_type=str, choices=["1", "2", "3"])

Foo(args="bar") #should raise an Error

Have you tested this? If so, how?

Added tests

[Tarrasch]

Nice fix. Thanks!