Generalize loadConfig method to avoid reading from disk (#5819)

As we move to multiple config backends like local disk and etcd,
config file should not be read from the disk, instead the quick
package should load and verify for duplicate entries.

pkg/quick/encoding.go

@@ -136,6 +136,11 @@ func loadFileConfig(filename string, v interface{}) error {
 	if runtime.GOOS == "windows" {
 		fileData = []byte(strings.Replace(string(fileData), "\r\n", "\n", -1))
 	}
+
+	if err = checkDupJSONKeys(string(fileData)); err != nil {
+		return err
+	}
+
 	// Unmarshal file's content
 	return toUnmarshaller(filepath.Ext(filename))(fileData, v)
 }