Configure Reaper as StatefulSet

[emerkle826]

Once thelastpickle/cassandra-reaper#1487 is complete, we need to be able to configure Reaper as a StatefulSet (instead of a Deployment) when Reaper uses its Memory Storage backend. When configured with the memory storage backend, Reaper will not need a Cassandra database to store its data, nor will we need an instance of Reaper deployed in every K8s cluster running a Cassandra cluster that we want to manage repairs for.

Depends upon

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1. Add an option to persist the memory storage data using a local store thelastpickle/cassandra-reaper#1487
   done +1
   
2. Add env variable to Docker image for Memory Storage backend location thelastpickle/cassandra-reaper#1489
   done +1

Definition of Done

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1. Reaper backend is configurable (choice of cassandra or memory)
2. Reaper is deployed as a StatefulSet when configured with memory backend
3. Reaper is deployed as a Deployment when configured with cassandra backend (the current behavior)
4. Reaper memory storage location is configurable and should specify a PVC
5. Reaper does not have an init container when deployed as a statefull set
6. Ensure there is always only one Reaper instance out there
7. Ensure the ReaperClusterTemplate.DeploymentMode is SINGLE when memory storage is used