Improvements to OpenStack inventory script
nova.py: Set defaults for OpenStack settings

- 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'
```
nova.py: Support OS_AUTH_SYSTEM and OS_REGION_NAME
nova.ini: Distinguish between required and optional settings

Put them in separate sections of config to make it more clear what is
essential and what is not. Also comment out the optional settings.
And remove duplicate mention of `service_type`.