Persist cassandra volume

fixes: GRADE-243

Allow cassandra's data to persist after using `docker-compose down
cassandra` where before this command would effectively wipe your database
This is in line with how the pg_data volumes work.

test plan:
- setup cassandra
- create a keyspace
- run `docker-compose down && docker-compose up -d` (-d for daemon)
- the previously create keyspace still exists

Change-Id: Ib58bec022eda79ad45352419e4fd33b07af7cf21
Reviewed-on: https://gerrit.instructure.com/124278
Tested-by: Jenkins
Reviewed-by: Shahbaz Javeed <sjaveed@instructure.com>
Reviewed-by: Matt Taylor <mtaylor@instructure.com>
QA-Review: KC Naegle <knaegle@instructure.com>
Product-Review: Derek Bender <djbender@instructure.com>

docker-compose/cassandra.override.yml

@@ -11,5 +11,10 @@ services:
 
   cassandra:
     image: cassandra:2.2
+    volumes:
+      - cassandra_data:/var/lib/cassandra
     environment:
       CASSANDRA_START_RPC: 'true'
+
+volumes:
+  cassandra_data: {}