mqtt output: cleanup, implement TLS

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

plugins/outputs/cloudwatch/cloudwatch.go

@@ -25,10 +25,10 @@ type CloudWatch struct {
 }
 
 var sampleConfig = `
-  # Amazon REGION
+  ### Amazon REGION
   region = 'us-east-1'
 
-  # Namespace for the CloudWatch MetricDatums
+  ### Namespace for the CloudWatch MetricDatums
   namespace = 'InfluxData/Telegraf'
 `