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/components.go

@@ -0,0 +1,31 @@
+/*
+ * 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
+
+type component string
+
+// Component names for debug output
+const (
+	NET component = "[net]     "
+	PNG component = "[pinger]  "
+	CLI component = "[client]  "
+	DEC component = "[decode]  "
+	MES component = "[message] "
+	STR component = "[store]   "
+	MID component = "[msgids]  "
+	TST component = "[test]    "
+	STA component = "[state]   "
+	ERR component = "[error]   "
+)