cloud storage tar file creation issue
<!-- If you don't know your Rasa NLU version, here is some help: https://goo.gl/g9QQg2. If you are creating a feature request, feel free to remove all the system information stuff. --> 

**Rasa NLU version** (e.g. `0.7.3`): 
0.12.0a2
**Used backend / pipeline** (`mitie`, `spacy_sklearn`, ...): 
spacy_sklearn
**Operating system** (windows, osx, ...): 
osx
**Issue**:
Issue with tar file created when cloud storage is turned on.  

Believe it has something to do with tempfile.mkdtemp() command added to _compress in persistor.py here:

```
        dirpath = tempfile.mkdtemp()
        base_name = self._tar_name(model_name, project, include_extension=False)
        tar_name = shutil.make_archive(base_name, 'gztar',
                                       root_dir=model_directory,
                                       base_dir=dirpath)
```
When running this version of the code with cloud storage turned on the tar file produced has a temporary folder structure inside like this:

`var/folders/vx/559bbyyj6b3dztyyyyp2mg6sgmmn5d/T/tmpF1VInL/`

If I replace this with code like is in the current released version:

```
        base_name = self._tar_name(model_name, project, include_extension=False)
        tar_name = shutil.make_archive(base_name, 'gztar',
                                       root_dir=model_directory,
                                       base_dir=".")
```

The contents of the tar file are as I would expect:

```
./
./training_data.json
./intent_classifier_sklearn.pkl
./metadata.json
./entity_synonyms.json
./regex_featurizer.json
./crf_model.pkl
```

**Content of configuration file** (if used & relevant):
```json

```

diff --git a/rasa_nlu/persistor.py b/rasa_nlu/persistor.py
index e94f0e26231..d53eefee6c6 100644
--- a/rasa_nlu/persistor.py
+++ b/rasa_nlu/persistor.py
@@ -89,9 +89,10 @@ class Persistor(object):
 
         dirpath = tempfile.mkdtemp()
         base_name = self._tar_name(model_name, project, include_extension=False)
-        tar_name = shutil.make_archive(base_name, 'gztar',
+        tar_name = shutil.make_archive(os.path.join(dirpath, base_name),
+                                       'gztar',
                                        root_dir=model_directory,
-                                       base_dir=dirpath)
+                                       base_dir=".")
         file_key = os.path.basename(tar_name)
         return file_key, tar_name
 
['rasa_nlu/persistor.py']