Support Zookeeperless NiFi

[nightkr]

Description

Fixes #737.

This makes spec.clusterConfig.zookeeperConfigMapName optional, switching to the Kubernetes provider if it is not set. The prose docs mention both options, but the quick start guide ("First steps") sticks to ZooKeeper, since it still also recommends NiFi 1.x.

A potential v1alpha2 could change this to something like

spec:
  clusterConfig:
    clusterBackend:
      zookeeper:
        configMapName: ...
      # or...
      kubernetes: {}

But I don't think it's worth the compatibility break for v1alpha1.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

# Author
- [X] Changes are OpenShift compatible
- [X] CRD changes approved
- [X] CRD documentation for all fields, following the [style guide](https://docs.stackable.tech/home/nightly/contributor/docs/style-guide).
- [X] Helm chart can be installed and deployed operator works
- [X] Integration tests passed (for non trivial changes)
- [X] Changes need to be "offline" compatible

# Reviewer
- [ ] Code contains useful comments
- [ ] Code contains useful logging statements
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated. Follows the [style guide](https://docs.stackable.tech/home/nightly/contributor/docs/style-guide).
- [ ] Changelog updated
- [ ] Cargo.toml only contains references to git tags (not specific commits or branches)

# Acceptance
- [ ] Feature Tracker has been updated
- [ ] Proper release label has been added
- [ ] [Roadmap](https://github.com/orgs/stackabletech/projects/25/views/1) has been updated

[nightkr]

Moving the CRD changes to RfC.

[nightkr]

Moving the CRD changes to voting.

[NickLarsenNZ]

Sorry, didn't see that it was already RfC and now in voting

Sounds good to me. Moving to option is non-breaking.

What do we do in cases where it is None, but the NiFi version still requires it?
I assume we error out on reconcile.

[nightkr]

In that case NiFi would crash on startup. We can add an explicit check for it.. maybe a bit overkill but if we're keeping 1.x for the relatively long haul then it might make sense?

[sbernauer]

I think having a snafu Error like "zookeeper-less election not supported for version {}. It's only supported for NiFi 2.x" would be great!

[nightkr]

Added an error in 4f63520.

[nightkr]

Tests: https://testing.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/nifi-operator-it-custom/38/, https://testing.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/nifi-operator-it-custom/39/

[nightkr]

Moving the CRD change to approved.

All tests passed in Jenkins except for ldap_nifi-2.2.0_zookeeper-latest-3.9.3_ldap-use-tls-true_openshift-false, but that passed locally.