[WIP] download.sh: Allow provision to retrieve puppet modules from tar ball

Currently the download.sh script retrieves the puppet modules running
r10k. Unfortunately, we - as consumer - don't have full control about
what happens upstream, and if a git push -f comes in the way, the r10k
for a given release won't be working anymore. Since we already tar those
modules, we offer the user to use those tared modules to be sure a release
will always be working.
@fcharlier I have to disagree with you on that one.

```
[spredzy@laptop]/tmp/plop% curl http://example.com/spinal-stack/I.1.2.0/puppet-openstack-cloud-I.1.2.0.tgz | tar x 
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0tar: Archive is compressed. Use -z option
tar: Error is not recoverable: exiting now
  0 5093k    0 20708    0     0  47166      0  0:01:50 --:--:--  0:01:50 47278
curl: (23) Failed writing body (912 != 5236)
[spredzy@laptop]/tmp/plop% curl http://example.com/spinal-stack/I.1.2.0/puppet-openstack-cloud-I.1.2.0.tgz | tar xz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 5093k  100 5093k    0     0  1278k      0  0:00:03  0:00:03 --:--:-- 1278k
[spredzy@laptop]/tmp/plop%
```

If I missed something just let me know,