- auth_system
- region_name
- service_type

These are config settings that could be left out in many scenarios, but
the current code is requiring them. In particular, "service_type" is a
new one in PR #7444 so if we add that and don't set a default, then
existing .ini files won't work:

```
  File
"/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/ConfigParser.py",
line 618, in get
    raise NoOptionError(option, section)
ConfigParser.NoOptionError: No option 'service_type' in section:
'openstack'
```