Cloudinary per field configuration doesn't work

Hi!

**Describe the bug**
I'm using Cloudinary as my media library. I want to set `output_filename_only: false` for all fields globally and enable it on specific fields but it only works globally.

With the config below I always get the full url.

**Expected behavior**
I would expect to define a global value for how the output of files should look like and change it on specific fields. At least thats how I understand the documentation.

> The Cloudinary media library integration can be configured in two ways: globally and per field
https://www.netlifycms.org/docs/cloudinary/#cloudinary-configuration-options

In my example I'd expect a full url in the body field and only the file name in the image field

**Applicable Versions:**

- Netlify CMS version: netlify-cms 2.3.3
- Git provider:GitHub
- OS: macOS High Sierra
- Browser version Firefox Nightlx

**CMS configuration**
```
media_library:
name: cloudinary
output_filename_only: false
config:
cloud_name: my_name
api_key: 1234567890

collections:
- name: 'til'
label: 'TIL'
folder: 'til'
create: true
slug: '{{slug}}'
fields:
- { label: 'Body', name: 'body', widget: 'markdown', required: false }
- { label: 'Image', name: 'image', widget: 'image', required: false, media_library: { output_filename_only: true } }
```

Thank you!