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/xl-v1-common_test.go

@@ -25,12 +25,6 @@ import (
 
 // Tests for if parent directory is object
 func TestXLParentDirIsObject(t *testing.T) {
-	rootPath, err := newTestConfig(globalMinioDefaultRegion)
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll(rootPath)
-
 	obj, fsDisks, err := prepareXL16()
 	if err != nil {
 		t.Fatalf("Unable to initialize 'XL' object layer.")