[network-topology] Replace DeviceNode data migration with management command

[nemesifier]

When using the integration with openwisp-controller, instead of using a data migration to create the initial DeviceNode objects, we should use a management command, because the logic to create the initial DeviceNode objects is in a method which cannot be used during a migration because concrete models cannot be used during data migrations without risking future issues when adding/removing fields.

We'll also need to add a few lines in the readme regarding this command (when to use it and an example).