Add support for MQTT server as a notification target (#4474)

This implementation is similar to AMQP notifications:

* Notifications are published on a single topic as a JSON feed
* Topic is configurable, as is the QoS. Uses the paho.mqtt.golang
  library for the mqtt connection, and supports connections over tcp
  and websockets, with optional secure tls support.
* Additionally the minio server configuration has been bumped up
  so mqtt configuration can be added.
* Configuration migration code is added with tests.

MQTT is an ISO standard M2M/IoT messaging protocol and was
originally designed for applications for limited bandwidth
networks. Today it's use is growing in the IoT space.

vendor/github.com/eclipse/paho.mqtt.golang/options_reader.go

@@ -0,0 +1,132 @@
+/*
+ * Copyright (c) 2013 IBM Corp.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ *
+ * Contributors:
+ *    Seth Hoenig
+ *    Allan Stockdill-Mander
+ *    Mike Robertson
+ */
+
+package mqtt
+
+import (
+	"crypto/tls"
+	"net/url"
+	"time"
+)
+
+// ClientOptionsReader provides an interface for reading ClientOptions after the client has been initialized.
+type ClientOptionsReader struct {
+	options *ClientOptions
+}
+
+func (r *ClientOptionsReader) Servers() []*url.URL {
+	s := make([]*url.URL, len(r.options.Servers))
+
+	for i, u := range r.options.Servers {
+		nu := *u
+		s[i] = &nu
+	}
+
+	return s
+}
+
+func (r *ClientOptionsReader) ClientID() string {
+	s := r.options.ClientID
+	return s
+}
+
+func (r *ClientOptionsReader) Username() string {
+	s := r.options.Username
+	return s
+}
+
+func (r *ClientOptionsReader) Password() string {
+	s := r.options.Password
+	return s
+}
+
+func (r *ClientOptionsReader) CleanSession() bool {
+	s := r.options.CleanSession
+	return s
+}
+
+func (r *ClientOptionsReader) Order() bool {
+	s := r.options.Order
+	return s
+}
+
+func (r *ClientOptionsReader) WillEnabled() bool {
+	s := r.options.WillEnabled
+	return s
+}
+
+func (r *ClientOptionsReader) WillTopic() string {
+	s := r.options.WillTopic
+	return s
+}
+
+func (r *ClientOptionsReader) WillPayload() []byte {
+	s := r.options.WillPayload
+	return s
+}
+
+func (r *ClientOptionsReader) WillQos() byte {
+	s := r.options.WillQos
+	return s
+}
+
+func (r *ClientOptionsReader) WillRetained() bool {
+	s := r.options.WillRetained
+	return s
+}
+
+func (r *ClientOptionsReader) ProtocolVersion() uint {
+	s := r.options.ProtocolVersion
+	return s
+}
+
+func (r *ClientOptionsReader) TLSConfig() tls.Config {
+	s := r.options.TLSConfig
+	return s
+}
+
+func (r *ClientOptionsReader) KeepAlive() time.Duration {
+	s := r.options.KeepAlive
+	return s
+}
+
+func (r *ClientOptionsReader) PingTimeout() time.Duration {
+	s := r.options.PingTimeout
+	return s
+}
+
+func (r *ClientOptionsReader) ConnectTimeout() time.Duration {
+	s := r.options.ConnectTimeout
+	return s
+}
+
+func (r *ClientOptionsReader) MaxReconnectInterval() time.Duration {
+	s := r.options.MaxReconnectInterval
+	return s
+}
+
+func (r *ClientOptionsReader) AutoReconnect() bool {
+	s := r.options.AutoReconnect
+	return s
+}
+
+func (r *ClientOptionsReader) WriteTimeout() time.Duration {
+	s := r.options.WriteTimeout
+	return s
+}
+
+func (r *ClientOptionsReader) MessageChannelDepth() uint {
+	s := r.options.MessageChannelDepth
+	return s
+}