change AsyncTaskList to use generic external task when executing tasks

@drsnyder @pchojnacki 
Changes to the MW app to use the generic external worker in celery-workers. The changes here are mostly to convert task args to the new expected format, and to support --wiki_id as a command line arg for ```task_runner.php```, since the local and remote shell execution methods don't specifically set the SERVER_ID environment var.

See https://github.com/Wikia/celery-workers/pull/23 and https://github.com/Wikia/flower/pull/3

*Note* - This PR shouldn't be merged until the celery-workers and flower PRs (above) are merged
Can we use `$this->mOptions['wiki_id']` here and get rid of `$wgCityId`?