Support Cassandra as Deep Storage for Druid.

- 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
- Leaves Cassandra at version 1.1.2 because Druid does not yet support
  2.x, but adds documenting comments in relevant source files about how
  to update to 2.x once Druid does support it.