nova: refactor the IronicUnderstackDriver to make testing easier

This way our modified code is almost all in separate method and it's
clear what has been changed.

Author: skrobul

python/nova-understack/ironic_understack/driver.py

@@ -114,7 +114,9 @@ def _generate_configdrive(
             extra_md = {}
 
         ### Understack modified code START
-        network_metadata = self._understack_get_network_metadata(instance, node, network_info)
+        network_metadata = self._understack_get_network_metadata(
+            instance, node, network_info
+        )
         ### Understack modified code END
 
         i_meta = instance_metadata.InstanceMetadata(