Changing CassandraCluster persistence settings should not be allowed

[wallrj]

Right now, if you change the persistence settings for a running cluster, the navigator controller will attempt to update the statefulset with the new volumeclaimtemplate settings.

I'm not sure whether this is supported and even if so, Navigator doesn't yet have a way to control the rollout of the updated statefulset.

We should prevent the user from changing the persistence setting for now.

See:

• https://github.com/kubernetes/kubernetes/issues/41015
• https://github.com/kubernetes/kubernetes/pull/46669
• But https://github.com/kubernetes/community/pull/503 suggests that updating storage settings was out of scope.
• https://github.com/kubernetes/kubernetes/issues/35941

/kind bug

[wallrj]

This shouldn't happen after #256 but we can add validation to make the persistence setting immutable. Similar to what @kragniz did for Rack and DC