ISPN-4868 Release.py script fails to copy artifacts

https://issues.jboss.org/browse/ISPN-4868
> Wouldn't this be equivalent to setting multi_threaded = false in the settings?

Setting ```multi_threaded = false``` would case steps 5, 6, 7.1, 7.2, 7.3, 7.4 and 8 to run sequentially. 
The code above will make 7.1 - 7.4 sequential, the rest is still running in multiple processes in parallel.

> I think we can create a different temp dir for each dist_dir instead.

Yes, sounds better than locking, let me give a try