swf: WorkflowTask: add the possibility to use a custom task list

[ThomasMagalhaes]

This is meant to be used by defining a function:

    @classmethod
    def get_task_list(cls, task_list, *args, **kwargs):
        return task_list

in the class implementing the Workflow class and allows then to use something like:

chain.append(
    MyWorkflow,
    param1=toto,
    param2=tata,
    task_list="the_task_list",
)