Migrate config.json from config-dir to backend (#6195)

This PR is the first set of changes to move the config
to the backend, the changes use the existing `config.json`
allows it to be migrated such that we can save it in on
backend disks.

In future releases, we will slowly migrate out of the
current architecture.

Fixes #6182

cmd/globals.go

@@ -126,6 +126,9 @@ var (
 	// Holds the host that was passed using --address
 	globalMinioHost = ""
 
+	// globalConfigSys server config system.
+	globalConfigSys *ConfigSys
+
 	globalNotificationSys *NotificationSys
 	globalPolicySys       *PolicySys