Prevent existing nodes from changing token ownership

[a1exsh]

Due to change in data directory layout in older versions of planb-cassandra, while updating the docker image a node may lose its system keyspace. What happens in this case is that the node joins the ring, but without the old tokens or data.

We should not start the Cassandra server if there is no data directory (no data/system), unless we know this is the new node. We can achieve that by providing a hint in the environment when creating new node in create_cluster. The update_cluster will take care to remove the hint before restarting the node.