mqtt output: cleanup, implement TLS

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

plugins/outputs/opentsdb/opentsdb.go

@@ -22,17 +22,17 @@ type OpenTSDB struct {
 }
 
 var sampleConfig = `
-  # prefix for metrics keys
+  ### prefix for metrics keys
   prefix = "my.specific.prefix."
 
   ## Telnet Mode ##
-  # DNS name of the OpenTSDB server in telnet mode
+  ### DNS name of the OpenTSDB server in telnet mode
   host = "opentsdb.example.com"
 
-  # Port of the OpenTSDB server in telnet mode
+  ### Port of the OpenTSDB server in telnet mode
   port = 4242
 
-  # Debug true - Prints OpenTSDB communication
+  ### Debug true - Prints OpenTSDB communication
   debug = false
 `