Refactor of WrapperTask to avoid repated complete() calls

Instead uses the in-memory mock file system in MockFile to keep state
and rely on luigi's task execution order to assume task completeness
of dependencies
It's not printing to console, it's printing to a fake file object in the MockFileSystem, so it's not really logging. It wouldn't have to write anything actually, it will only ever check that the file exists, but if anyone would read from the mock file system I thought "completed" would be suitable content of that "file".