Persist cassandra volume
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 <6692d7be3fa215e5483a121d6fb314def6049877@instructure.com>
Reviewed-by: Matt Taylor <6523858f0187e99881b3902a31b8a85366f3df97@instructure.com>
QA-Review: KC Naegle <9fbecd016338f9910821a8a5d5d0ae4fc9abfaa5@instructure.com>
Product-Review: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>

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