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!

Comienza con GCENodeDriver.ex_create_multiple_nodes y sigue cómo se producen los fallos de timeout después de que se hayan creado algunos nodos. El issue no decide entre eliminar esos nodos, exponerlos mediante la excepción o hacer yield de ellos; aclara primero esa decisión de API y, después, define done como garantizar que los llamadores puedan gestionar los nodos ya creados sin dejarlos inaccesibles.

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