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`.

plugins/inventory/nova.ini

 # Ansible OpenStack external inventory script
 
 [openstack]
+
+#-------------------------------------------------------------------------
+#  Required settings
+#-------------------------------------------------------------------------
+
 # API version
 version       = 2
 
 # OpenStack nova username
 username      =
 
-# OpenStack nova api_key
+# OpenStack nova api_key or password
 api_key       =
 
 # OpenStack nova auth_url
 auth_url      =
 
-# Authentication system
-auth_system = keystone
+# OpenStack nova project_id or tenant name
+project_id    =
 
-# OpenStack nova project_id
-project_id    = 
+#-------------------------------------------------------------------------
+#  Optional settings
+#-------------------------------------------------------------------------
+
+# Authentication system
+# auth_system = keystone
 
 # Serverarm region name to use
-region_name   =
+# region_name   =
 
 # Specify a preference for public or private IPs (public is default)
-prefer_private = False
+# prefer_private = False
 
 # What service type (required for newer nova client)
-service_type = compute
+# service_type = compute
 
 
 # TODO: Some other options
 # insecure      =
 # endpoint_type =
 # extensions    =
-# service_type  =
 # service_name  =