EBS volume duplication

[SergioGonzalezSanz]

During the cluster creation, the function create_cluster.launch_instance creates a new EBS volumen per each Cassandra node without checking first if a volume exists with the same name. Then, a volume with that name is attached to the node. It can cause 'volume duplication' in the sense that there will be two or more EBS volumes with the same name (not necessarily the same data) and it will make the instance-EBS volume attachment indeterministic.
If we have a set of previously existing EBS volumes from an old cluster and we want to use the existing data in the new Cassandra cluster, the script should check first if a volumen with the given name exists and if it does, it shouldn't create it but just attach it to the instance.
It is also really useful for implementing a failover strategy for Cassandra. It allows us to make snapshots of our EBS volumes and in the case that something happens, we can recreate a new cluster from the snapshots since they will be attached to the new instances.