mqtt output: cleanup, implement TLS

Also normalize TLS config across all output plugins and normalize
comment strings as well.

plugins/outputs/nsq/nsq.go

@@ -14,9 +14,9 @@ type NSQ struct {
 }
 
 var sampleConfig = `
-  # Location of nsqd instance listening on TCP
+  ### Location of nsqd instance listening on TCP
   server = "localhost:4150"
-  # NSQ topic for producer messages
+  ### NSQ topic for producer messages
   topic = "telegraf"
 `