mqtt output: cleanup, implement TLS

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

plugins/outputs/amon/amon.go

@@ -22,13 +22,13 @@ type Amon struct {
 }
 
 var sampleConfig = `
-  # Amon Server Key
+  ### Amon Server Key
   server_key = "my-server-key" # required.
 
-  # Amon Instance URL
+  ### Amon Instance URL
   amon_instance = "https://youramoninstance" # required
 
-  # Connection timeout.
+  ### Connection timeout.
   # timeout = "5s"
 `