Key name was changed in version 2 of the digital ocean API

Keyname has changed in version 2 of digital oceans API

```
[DEBUG   ] Failed to execute 'digital_ocean.list_nodes()' while querying for running nodes: 'size'
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/salt/cloud/__init__.py", line 2195, in run_parallel_map_providers_query
    cloud.clouds[data['fun']]()
  File "/usr/lib/python2.7/site-packages/salt/cloud/clouds/digital_ocean_v2.py", line 158, in list_nodes
    'size': node['size']['slug'],
KeyError: 'size'
```