Allow performing restores without having a running cluster

[adejanovski]

Cassandra data should be restore-able on the initial cluster startup. The K8ssandraCluster object would need to be augmented with settings that indicate which backup should be restored on the first start:

    datacenters:
      - metadata:
          name: dc1
        k8sContext: dev
        size: 3
        restoreFrom:
          backupName: medusa_20220315

Connection settings would still be set up in the Medusa section of the manifest, as Medusa's containers will rely on it. The prefix will not be customizable for the restore operation in this first version due to the amount of necessary changes. This would be implemented in a follow up set of issues.

This work will also leverage what will be done in #486.

Here's the sequence of events that should happen:

• Reconcile starts
• The Medusa Standalone pod gets created
• The operator can read the requested backup through the above pod, compute the mapping and store it in a configMap
• The cassdc config is updated with the necessary bits to get the restore done on the first start (including the restore mapping config map mount)
• The statefulsets will get created and the restore happens in the medusa-restore init-container
• Reconciliation goes through unless restore failed (which means that the pods will be crash looping)

The initial restore should happen only once, all subsequent restores happening through a classic MedusaRestoreJob. The reconcile should check if the k8c has the CassandraInitialized condition set to true and skip the restore if it's the case.

┆Issue is synchronized with this Jira Story by Unito ┆Issue Number: K8OP-149

[bradfordcp]

Hey team! Please add your planning poker estimate with ZenHub @adejanovski @adutra

[adejanovski]

@jsanda @burmanm, we may want to have a quick design session before moving on with this. I feel like we're not totally on the same page on how this should be implemented.

[jsanda]

Agreed about the design session. Let's make sure we are all on the same page and get it documented.