Cassandra data should be persisted to disk

The Cassandra statefulset / pods should have a persistent disk.

See https://github.com/jetstack/navigator/blob/master/pkg/controllers/elasticsearch/nodepool/resources.go#L110

if !np.Persistence.Enabled {
    volumes = append(volumes, apiv1.Volume{
        Name: esDataVolumeName,
        VolumeSource: apiv1.VolumeSource{
            EmptyDir: &apiv1.EmptyDirVolumeSource{},
        },
    })
}

/kind feature

jetstack / navigator

Cassandra data should be persisted to disk #216