Switch Cassandra test hosts to the new role class

- Move system::role under the role class.
- Get rid of the vm.max_map_count sysctl, included in Cassandra's Debian
  package now.
- Split ruthenium off the host list, to be repurposed.

Change-Id: I3e5a4b95a906fd9797a731cafc80d8d94ca45db9