mqtt output: cleanup, implement TLS

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

plugins/outputs/datadog/datadog.go

@@ -24,10 +24,10 @@ type Datadog struct {
 }
 
 var sampleConfig = `
-  # Datadog API key
+  ### Datadog API key
   apikey = "my-secret-key" # required.
 
-  # Connection timeout.
+  ### Connection timeout.
   # timeout = "5s"
 `