[API] Tasks.list spec doesn't reflect reality

[honzakral]

https://github.com/elastic/elasticsearch/blob/master/rest-api-spec/src/main/resources/rest-api-spec/api/tasks.list.json specifies params node_id and parent_task instead of accepted nodes and parent_task_id.

This is present in 5.6, 6.0, and master branches.

Thanks, @nfsec, for discovering this in elastic/elasticsearch-py#659

[olcbean]

I'd like to give this one a try.
It looks like the parameter names have been changed (in the code and in the docs, but not in the rest-api-spec). As the tests do not use these parameters, no error has been detected till now.