Parallelization improvements

Using this patch, we can now get the output from many parellized creates when running from a map file. The mods are all around the multiprocessing code.

Currently salt-cloud will start as many processes as there are new VMs to start - (this patch also behaves this way) - but I'm not sure if that's the best approach. Perhaps someone more knowledgable in this area can offer some wisdom..

Some benchmarks creating instances in EC2 from an existing m1.small VM in the same region:

```
servers	secs
3		206
6		270
8		385
12		509
15		605
20		641
```

NB: this patch doesn't include anything around the `tty` flag to false when calling `deploy_script`. I need `tty` to be false to prevent my terminal getting mangled during the create.