Updated MQTT docs to include api key substitution

Author: tomczoink

content/concepts/mqtt.textile

@@ -49,8 +49,8 @@ An example of how you might configure an MQTT client to connect through an "MQTT
 ```[javascript]
 var options = {
   keepalive: 60,
-  username: 'FIRST_HALF_OF_API_KEY',
-  password: 'SECOND_HALF_OF_API_KEY',
+  username: '{{API_KEY_NAME}}',
+  password: '{{API_KEY_SECRET}}',
   port: 8883
 };
 var client = mqtt.connect('mqtts:mqtt.ably.io', options);
@@ -59,7 +59,7 @@ var client = mqtt.connect('mqtts:mqtt.ably.io', options);
 Any data published through or received by an MQTT broker will be binary encoded as MQTT is a binary protocol. This means that you’ll need to interpret the message to get the original contents out. Here’s how it looks using Ably and JavaScript:
 
 ```[javascript]
-var ably = new Ably.Realtime('REPLACE_WITH_YOUR_API_KEY');
+var ably = new Ably.Realtime('{{API_KEY}}');
 var decoder = new TextDecoder();
 var channel = ably.channels.get('input');
 channel.subscribe(function(message) {

content/mqtt/index.textile

@@ -74,7 +74,7 @@ h2(#decoding). Decoding MQTT communication
 Any data published through or received by the MQTT adapter will be binary encoded, due to MQTT being a binary protocol. This means that you'll need to interpret the message to get the original contents out. For example, to interpret a message using "Ably Realtime":https://www.ably.io/documentation/realtime with JavaScript you'd need to do the following, using the "text-encoding NPM module's TextDecoder":https://www.npmjs.com/package/text-encoding to decode from binary to text:
 
 bc[javascript]. {
-  var ably = new Ably.Realtime('REPLACE_WITH_YOUR_API_KEY');
+  var ably = new Ably.Realtime('{{API_KEY}}');
   var decoder = new TextDecoder();
   var channel = ably.channels.get('my_channel');
   channel.subscribe(function(message) {