Make Reaper's backend configurable

[rzvoncek]

What this PR does: This PR adds the option to configure Reaper to use a memory sotrage backend. If this storage type is used, Reaper will be deployed as a Stateful Set instead of a Deployment. It will also get a new volume to store the data on. This volume is not being cleared up after Reaper's parent k8ssandra cluster object gets deleted. This is intentional - we want to keep that data, plus it's a pre-work for deploying reaper individually (w/o the k8ssandra cluster).

Which issue(s) this PR fixes: Fixes #1275

Checklist

• [x] Changes manually tested
• [x] Automated Tests added/updated
• [ ] Documentation added/updated
• [x] CHANGELOG.md updated (not required for documentation PRs)
• [ ] CLA Signed: DataStax CLA