Support Cassandra as Deep Storage for Druid.

The default is still to use S3 as Deep Storage, however this changeset allows using Cassandra as an alternative. Simply edit the configure_*.sh scripts to enable Cassandra and disable S3.

- Updates Cassandra to version 2.0.9.
- Adds Cassandra dependency to Druid service.
- Resolves issues related to getRole() usage.
- Adds druid-cassandra node role to Druid cluster.
- Cleans up Druid configuration scripts.
- Fixes issue in Druid install script, was never installing the requested Druid version and instead always using the default.
- Fixes issue in Cassandra install script, was never installing the requested Cassandra version and instead always using the default.
- Updates default Druid version to 0.6.121.1