cassandra: Add a template configuration file

Add a template configuration file that is picked up by OSv's cloud-init.

Cassandra uses multi-region EC2 snitch with the following options
auto-configured:

  - Cluster name
  - Listen addresses
  - Broadcast addresses

Seed nodes only contains the external IP of the node so you can only
start one node in a cluster. The plan is to extend cloud-init Cassandra
configuration module to use DataStax reflector for this.

NOTE! We also package a default cassandra.yaml which will be used on
platforms that don't have cloud-init like Capstan.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>