nova.py: Set defaults for OpenStack settings

- auth_system
- region_name

These are config settings that could be left out in many scenarios, but
the current code is requiring them. With this in place, I can get by
with this minimal .ini file:

```ini
[openstack]
version=1.1
auth_url=http://{keystone_host}:5000/v2.0
project_id=web
username={username}
api_key={password}
```
Merge pull request #8007 from msabramo/nova.py_set_defaults

nova.py: Set defaults for OpenStack settings