[Feature/Operator] Enable resource lifecycle management for external NiFi cluster

[erdrix]

Q	A
Bug fix?	yes
New feature?	yes
API breaks?	no
Deprecations?	no
Related tickets	fixes #131
License	Apache 2.0

What's in this PR?

The main focus of this feature is to provide a way to manager the dataflow lifecycle on external cluster (ie. cluster not created using the NifiCluster kubernetes resource). It allow to deploy NifiDataflow, NifiUser, NifiUserGroup, NifiParameterContext and NifiRegistryClient on clusters that are not managed by the operator using a TLS or basic authentication.

This PR also include some fix and new features :

• Fix the PR Failing Nifi cluster downscale because missing configmap #131 by removing references to configmap
• Improving the interaction with the clusters by checking it status
• Enabling the ability to move a resource from one cluster to another by just changing the clusterReference.

Why?

Like the configConnector approach, some resources managed by the operator are not directly related to kubernetes, but just some interaction with the NIFI cluster API. So, we want to enable all the dataflow lifecycle logic even if people don't want to manage their cluster using the operator.

Checklist

• Implementation tested
• Error handling code meets the guideline
• Logging code meets the guideline
• User guide and development docs updated (if needed)
• Append changelog with changes