Better Nifi cluster status api

[razvan]

Issue checklist

We now have a ClusterIP service called <nifi-cluster-name>-reporting-task that always points to a single Nifi pod in the StatefulSet. This is used to query the Nifi cluster status and ensures that the Nifi JWT token can be validated by the same pod that generated it.

This was implemented here and was more of a quick workaround.

What we should have:

• a better name for the service. Proposal: <nifi-cluster-name>-cluster-status.
• this service should be configurable as external so that clients outside the K8S cluster can query it too. The getting started guide would need it.
• optional: maybe a more robust routing in case the said pod becomes unavailable. This very much depends on Nifi's implementation of authorization.