mqtt output: cleanup, implement TLS

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

plugins/outputs/graphite/graphite.go

@@ -22,11 +22,11 @@ type Graphite struct {
 }
 
 var sampleConfig = `
-  # TCP endpoint for your graphite instance.
+  ### TCP endpoint for your graphite instance.
   servers = ["localhost:2003"]
-  # Prefix metrics name
+  ### Prefix metrics name
   prefix = ""
-  # timeout in seconds for the write connection to graphite
+  ### timeout in seconds for the write connection to graphite
   timeout = 2
 `