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/handler-utils_test.go

@@ -30,11 +30,14 @@ import (
 
 // Tests validate bucket LocationConstraint.
 func TestIsValidLocationContraint(t *testing.T) {
-	path, err := newTestConfig(globalMinioDefaultRegion)
+	obj, fsDir, err := prepareFS()
 	if err != nil {
-		t.Fatalf("unable initialize config file, %s", err)
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(fsDir)
+	if err = newTestConfig(globalMinioDefaultRegion, obj); err != nil {
+		t.Fatal(err)
 	}
-	defer os.RemoveAll(path)
 
 	// Test with corrupted XML
 	malformedReq := &http.Request{