GCENodeDriver.ex_create_multiple_nodes doesn't clean itself when fails.

## Summary

When calling `GCENodeDriver.ex_create_multiple_nodes`, if it gets timeout, the whole operation raises an exception, and the already created nodes are dangling and are not passed to the caller.

## Detailed Information

When calling `GCENodeDriver.ex_create_multiple_nodes`, if it gets timeout, the whole operation raises an exception , and the already created nodes are dangling and are not passed to the caller. I think that actually deleting the created nodes is pretty intrusive. But, it may be helpful to pass the information regarding the already created nodes in the raised exception object, so that the caller would be able to act on this information. Another option is to yield all created nodes as soon as they are created.

Thanks!

Inizia da GCENodeDriver.ex_create_multiple_nodes e traccia come vengono sollevati gli errori di timeout dopo che alcuni nodi sono stati creati. L’issue non sceglie tra eliminare quei nodi, esporli tramite l’eccezione o restituirli con yield; chiarisci prima questa decisione dell’API, quindi definisci done come la garanzia che i chiamanti possano gestire i nodi già creati senza lasciarli inaccessibili.

Scritto dal modello di indicizzazione a partire dal testo della issue.