12341: If a node_destroy call succesfully destroys the VM but takes too long

12341: If a node_destroy call succesfully destroys the VM but takes too long
to finish with additional tasks, the cloud_node_update may be called
in the meantime, removing the node from the cloud_node list. If this happens,
node_finished_shutdown() won't be able to retrieve the node, and shouldn't
raise an exception.

Arvados-DCO-1.1-Signed-off-by: Lucas Di Pentima <ldipentima@veritasgenetics.com>