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/signature-v4_test.go

@@ -92,11 +92,14 @@ func TestDoesPolicySignatureMatch(t *testing.T) {
 }
 
 func TestDoesPresignedSignatureMatch(t *testing.T) {
-	rootPath, err := newTestConfig(globalMinioDefaultRegion)
+	obj, fsDir, err := prepareFS()
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer os.RemoveAll(rootPath)
+	defer os.RemoveAll(fsDir)
+	if err = newTestConfig(globalMinioDefaultRegion, obj); err != nil {
+		t.Fatal(err)
+	}
 
 	// sha256 hash of "payload"
 	payloadSHA256 := "239f59ed55e737c77147cf55ad0c1b030b6d7ee748a7426952f9b852d5a935e5"