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/trace.go

@@ -0,0 +1,36 @@
+/*
+ * 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 (
+	"io/ioutil"
+	"log"
+)
+
+// Internal levels of library output that are initialised to not print
+// anything but can be overridden by programmer
+var (
+	ERROR    *log.Logger
+	CRITICAL *log.Logger
+	WARN     *log.Logger
+	DEBUG    *log.Logger
+)
+
+func init() {
+	ERROR = log.New(ioutil.Discard, "", 0)
+	CRITICAL = log.New(ioutil.Discard, "", 0)
+	WARN = log.New(ioutil.Discard, "", 0)
+	DEBUG = log.New(ioutil.Discard, "", 0)
+}