Prevent reconciling on more than one NATS-CR

[k15r]

Description
At the moment, we support reconciling multiple instances of the NATS CR. For now, we will limit this functionality to only support reconciling a single instance (with a specific name within kyma-system namespace). All other instances of the NATS CR will not get ready and will have a clear condition telling the user about the limitation.

Questions

• Should we use admission webhook?
  • We cannot use admission webhook, because the lifecycle manager may deploy the nats-manager and CR at the same time. And if we have an admission webhook, it will not allow to create the CR until the webhook is ready.
  • Also, we will follow the convention of accepting multiple CRs, and reflecting the error in conditions. It's done like this in other module-managers.

Hint
In reconciler, get name and namespace, look if values are not allowed, set state to error, but pass reconciliation. Have a flag to disable this feature for tests.

Acceptance

• Make the name and namespace of the CR configurable in NATS-Manager.
• limit reconciling NATS CRs to one specific instance
• add a condition with a clear message to inform the user about the limitation
• set other instances of NATS CR to an error state
• integration test should still be able to create nats cr with different names & namespaces

[mfaizanse]