Migrate all backend at .minio.sys/config to encrypted backend (#8474)

- Supports migrating only when the credential ENVs are set,
  so any FS mode deployments which do not have ENVs set will
  continue to remain as is.
- Credential ENVs can be rotated using MINIO_ACCESS_KEY_OLD
  and MINIO_SECRET_KEY_OLD envs, in such scenarios it allowed
  to rotate the encrypted content to a new admin key.

cmd/globals.go

@@ -186,6 +186,9 @@ var (
 
 	globalActiveCred auth.Credentials
 
+	// Indicates if config is to be encrypted
+	globalConfigEncrypted bool
+
 	globalPublicCerts []*x509.Certificate
 
 	globalDomainNames []string      // Root domains for virtual host style requests