NATS Cluster name is not passed to Pods configuration

[rsafonseca]

Unlike helm chart deployments of NATS-server, it's not possible to effectively specify cluster name to the NATS-server pods.
Although this name is passed to the operator, it's only left in the CRD metadata and not included in the nats.conf, which results in these messages on Pod startup:

[1] 2021/03/23 10:56:56.609732 [INF] Cluster name is GmB9rA3nEw2Viajfz9rNXf
[1] 2021/03/23 10:56:56.609734 [WRN] Cluster name was dynamically generated, consider setting one

Additionally, when rolling out the nodes in a cluster for reasons such as upgrade, sometimes some pods get stuck in a loop logging this state:

[1] 2021/03/23 10:55:31.844349 [ERR] 10.136.107.23:49442 - rid:490 - Rejecting connection, cluster name "" does not match "iZkmDO3Ocr5CudBfxKNmZq"

#311 passes the cluster name to the pod's cluster config to avoid this.