Horizon Dashboard incompatible (partially) with Apple Safari 11.1 browser

Detailed bug description:

Confirmed in MOS 8.0 - MOS 9.2

Creating an instance from the Horizon dashboard results in the following error message:

Danger: There was an error submitting the form. Please try again.

Upon further review in the /var/log/horizon/horizon.log I find the following traceback:

```
2018-04-24 17:47:33,091 17773 ERROR django.request Internal Server Error: /horizon/project/instances/launch
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/django/core/handlers/base.py", line 104, in get_response
    response = middleware_method(request, callback, callback_args, callback_kwargs)
  File "/usr/lib/python2.7/dist-packages/django/middleware/csrf.py", line 174, in process_view
    request_csrf_token = request.POST.get('csrfmiddlewaretoken', '')
  File "/usr/lib/python2.7/dist-packages/django/core/handlers/wsgi.py", line 135, in _get_post
    self._load_post_and_files()
  File "/usr/lib/python2.7/dist-packages/django/http/request.py", line 233, in _load_post_and_files
    self._post, self._files = self.parse_file_upload(self.META, data)
  File "/usr/lib/python2.7/dist-packages/django/http/request.py", line 198, in parse_file_upload
    return parser.parse()
  File "/usr/lib/python2.7/dist-packages/django/http/multipartparser.py", line 146, in parse
    for item_type, meta_data, field_stream in Parser(stream, self._boundary):
  File "/usr/lib/python2.7/dist-packages/django/http/multipartparser.py", line 618, in __iter__
    for sub_stream in boundarystream:
  File "/usr/lib/python2.7/dist-packages/django/utils/six.py", line 535, in next
    return type(self).__next__(self)
  File "/usr/lib/python2.7/dist-packages/django/http/multipartparser.py", line 431, in __next__
    return LazyStream(BoundaryIter(self._stream, self._boundary))
  File "/usr/lib/python2.7/dist-packages/django/http/multipartparser.py", line 458, in __init__
    unused_char = self._stream.read(1)
  File "/usr/lib/python2.7/dist-packages/django/http/multipartparser.py", line 329, in read
    out = b''.join(parts())
  File "/usr/lib/python2.7/dist-packages/django/http/multipartparser.py", line 322, in parts
    chunk = next(self)
  File "/usr/lib/python2.7/dist-packages/django/utils/six.py", line 535, in next
    return type(self).__next__(self)
  File "/usr/lib/python2.7/dist-packages/django/http/multipartparser.py", line 344, in __next__
    output = next(self._producer)
  File "/usr/lib/python2.7/dist-packages/django/utils/six.py", line 535, in next
    return type(self).__next__(self)
  File "/usr/lib/python2.7/dist-packages/django/http/multipartparser.py", line 406, in __next__
    data = self.flo.read(self.chunk_size)
  File "/usr/lib/python2.7/dist-packages/django/http/request.py", line 267, in read
    six.reraise(UnreadablePostError, UnreadablePostError(*e.args), sys.exc_info()[2])
  File "/usr/lib/python2.7/dist-packages/django/http/request.py", line 265, in read
    return self._stream.read(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/core/handlers/wsgi.py", line 59, in read
    result = self.buffer + self._read_limited(size - len(self.buffer))
  File "/usr/lib/python2.7/dist-packages/django/core/handlers/wsgi.py", line 47, in _read_limited
    result = self.stream.read(size)
UnreadablePostError: Apache/mod_wsgi request data read error: Partial results are valid but processing is incomplete.
```

This is an issue for one particular customer as MacOS + Safari is their primary OS+Browser.

Steps to reproduce:

1. On Apple Safari Version 11.1 (13605.1.33.1.2) go to Horizon Dashboard.
2. Create instance.
3. Error produced.

Expected results:

1. Launching an instance should complete successfully.

Actual result:

See detailed bug description.

Reproducibility:

100% of the time.

Workaround:

User alternate browser or downgrade Safari.

Impact:
For most people this might not be an issue. However, customer requires use of Safari. Impact is high for them.