mqtt output: cleanup, implement TLS

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

plugins/outputs/prometheus_client/prometheus_client.go

@@ -16,7 +16,7 @@ type PrometheusClient struct {
 }
 
 var sampleConfig = `
-  # Address to listen on
+  ### Address to listen on
   # listen = ":9126"
 `
 

plugins/outputs/prometheus_client/prometheus_client_test.go

@@ -17,7 +17,8 @@ func TestPrometheusWritePointEmptyTag(t *testing.T) {
 		t.Skip("Skipping integration test in short mode")
 	}
 	pTesting = &PrometheusClient{Listen: "localhost:9127"}
-	pTesting.Start()
+	err := pTesting.Start()
+	require.NoError(t, err)
 	defer pTesting.Stop()
 
 	p := &prometheus.Prometheus{