Ganglia and persistent hdfs fixes

1.  Ganglia fixes include changing the storage directory to /mnt, deleting all the old data when a cluster starts up and reducing the amount of data stored (my estimate is that the data storage should be reduced at least by 10x)

2.  EBS volume setup now correctly detects if the volume has been already formatted / mounted etc. This should fix the issue reported at https://groups.google.com/d/msg/spark-users/ieOlGSuDd1c/_J0wW5IWOxkJ