Allow to retain namespace while operator deletion

[shtripat]

Using a command line flag --retain-namespace under command kubectl minio delete it would retain the namespace and delete all other resources.

$ kubectl minio delete minio --help 

'delete' command delete MinIO Operator along with all the tenants.

Usage:
  minio delete [flags]

Examples:
  kubectl minio delete

Flags:
  -d, --dangerous          confirm deletion
  -f, --force              allow without confirmation
  -h, --help               help for delete
  -n, --namespace string   namespace scope for this request (default "minio-operator")
  -r, --retain-namespace   retain operator namespace

Global Flags:

Fixes: #1112

Steps to verify changes

1. Deploy MinIO operator

$ kubectl minio init --namespace CUSTOM-NS

2. Remove MinIO operator with --retain-namespace flag

$ kubectl minio delete --namespace CUSTOM-NS -d -f -r

it should not delete the namespace
3. Repeat step-2 withou -r option and namespace should get deleted