Dont create default secret if existingSecret value set

[shtripat]

This allows to set an already created env configuration secret to the tenant. Set secrets.existingSecret to the name of pre created env configuration. Also value needs to be set at tenant.configuration.name to make sure tenant uses it.

Fixes: #1556

How to test the PR

1. Push these changes to a private helm repo
2. Create a local values.yaml as below content

## Secret with default environment variable configurations to be used by MinIO Tenant.
## Not recommended for production deployments! Create the secret manually instead.
secrets:
  name: myminio-env-configuration
  # MinIO root user and password
  accessKey: minio
  secretKey: minio123
  existingSecret: random-env-configuration

## MinIO Tenant Definition
tenant:
  # Tenant name
  name: myminio
  ## Registry location and Tag to download MinIO Server image
  image:
    repository: quay.io/minio/minio
    tag: RELEASE.2023-05-27T05-56-19Z
    pullPolicy: IfNotPresent
  ## Customize any private registry image pull secret.
  ## currently only one secret registry is supported
  imagePullSecret: { }
  ## If a scheduler is specified here, Tenant pods will be dispatched by specified scheduler.
  ## If not specified, the Tenant pods will be dispatched by default scheduler.
  scheduler: { }
  ## Secret name that contains additional environment variable configurations.
  ## The secret is expected to have a key named config.env containing environment variables exports.
  configuration:
    name: random-env-configuration
..........

3. Create a kind cluster
4. Deploy minio-operator

$ kubectl apply -k github.com/minio/operator

5. Create a custom env configuration secret.yaml with below content

apiVersion: v1
kind: Secret
metadata:
  name: random-env-configuration
  namespace: tenant-ns
type: Opaque
data:
  config.env: ZXhwb3J0IE1JTklPX1JPT1RfVVNFUj10ZXN0dXNlcgpleHBvcnQgTUlOSU9fUk9PVF9QQVNTV09SRD10ZXN0dXNlcjEyMwo=

where

$ echo "ZXhwb3J0IE1JTklPX1JPT1RfVVNFUj10ZXN0dXNlcgpleHBvcnQgTUlOSU9fUk9PVF9QQVNTV09SRD10ZXN0dXNlcjEyMwo=" | base64 -d
export MINIO_ROOT_USER=testuser
export MINIO_ROOT_PASSWORD=testuser123

$ kubectl create ns tenant-ns
$ kubectl appy -f secret.yaml

6. Deploy the minio tenant using local values.yaml from step-1

$ helm install minio1 --namespace tenant-ns <your-private-helm-repo>/tenant --values ./values.yaml

7. Verify that tenant pods come online properly

$ kubectl get pods -n tenant-ns
NAME               READY   STATUS    RESTARTS   AGE
myminio-pool-0-0   2/2     Running   0          19m
myminio-pool-0-1   2/2     Running   0          19m
myminio-pool-0-2   2/2     Running   0          19m
myminio-pool-0-3   2/2     Running   0          19m

8, Verify that the default tenant env configuration secret myminio-env-configuration is not crated

$ kubectl get secrets -n tenant-ns
NAME                           TYPE                 DATA   AGE
myminio-tls                    Opaque               2      21m
random-env-configuration       Opaque               1      21m
sh.helm.release.v1.minio1.v1   helm.sh/release.v1   1      21m