fix get csrSignerName from env MINIO_OPERATOR_CSR_SIGNER_NAME

[jiuker]

fix #1985
we should keep csrSingerName from env.

[pjuarezd]

may I suggest getDefaultCsrSignerName only returns a value if the env variable is set:

func getDefaultCsrSignerName() string {
         defaultCsrSignerNameOnce.Do(func() {
		if os.Getenv(CSRSignerName) != "" {
			defaultCsrSignerName = os.Getenv(CSRSignerName)
			return
		}
		defaultCsrSignerName = ""
	})
	return defaultCsrSignerName
}

If variable is set, we immediatly return that, else detect EKS, else return defalt CSR signer

func GetCSRSignerName(clientSet kubernetes.Interface) string {
	csrSignerNameOnce.Do(func() {
		csrSignerName = getDefaultCsrSignerName()
		if csrSignerName != "" {
		  return csrSignerName
		}
		// only for csr api v1 we will try to detect if we are running inside an EKS cluster and switch to AWS's way to
		// get certificates using their CSRSignerName https://docs.aws.amazon.com/eks/latest/userguide/cert-signing.html
		if GetCertificatesAPIVersion(clientSet) == CSRV1 {
			// if the user specified the EKS runtime, no need to do the check
			if utils.GetOperatorRuntime() == common.OperatorRuntimeEKS {
				csrSignerName = EKSCsrSignerName
				return
			}
			nodes, err := clientSet.CoreV1().Nodes().List(context.Background(), metav1.ListOptions{})
			if err != nil {
				klog.Infof("Could not retrieve nodes to determine if we are in EKS: %v", err)
			}
			// if we find a single node with a kubeletVersion that contains "eks" or a label that starts with
			// "eks.amazonaws.com", we'll start using AWS EKS signer name
			for _, n := range nodes.Items {
				if strings.Contains(n.Status.NodeInfo.KubeletVersion, "eks") {
					csrSignerName = EKSCsrSignerName
					break
				}
				for k := range n.ObjectMeta.Labels {
					if strings.HasPrefix(k, "eks.amazonaws.com") {
						csrSignerName = EKSCsrSignerName
					}
				}
			}
		}
		if csrSignerName == "" {
                     csrSignerName = certificatesV1.KubeletServingSignerName
                 }
	})
	return csrSignerName

[shtripat]

lgtm