Add admin get/set config keys API (#6113)

This PR adds two new admin APIs in Minio server and madmin package:
- GetConfigKeys(keys []string) ([]byte, error)
- SetConfigKeys(params map[string]string) (err error)

A key is a path in Minio configuration file, (e.g. notify.webhook.1)

The user will always send a string value when setting it in the config file,
the API will know how to convert the value to the appropriate type. The user
is also able to set a raw json.

Before setting a new config, Minio will validate all fields and try to connect
to notification targets if available.

pkg/madmin/examples/get-config.go

@@ -1,4 +1,6 @@
-/* +build ignore
+// +build ignore
+
+/*
  * Minio Cloud Storage, (C) 2017 Minio, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");