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'
```

plugins/inventory/nova.py

@@ -39,6 +39,7 @@ NOVA_CONFIG_FILES = [os.getcwd() + "/nova.ini",
 NOVA_DEFAULTS = {
     'auth_system': None,
     'region_name': None,
+    'service_type': 'compute',
 }