cassandralauncher doesn't check that DataStaxLauncher.pem matches

[mdennis]

cassandralauncher checks if .ssh/DataStaxLauncher.pem exists and has the correct permissions. If it doesn't, a new one is created. However, if one is copied from another use of cassandralauncher under a different account it continues to launch the cluster with no errors but the nodes are not correctly configured (again with no errors, opsc just hangs waiting for agents).

cassandralauncher should check that the fingerprint for the keypair on the file system matches the fingerprint for the keypair on the EC2 account being used and error out before a cluster launch is attempted (since otherwise it will launch the nodes but they won't be usable)