title: Get AZ of a subnet through cloudformation script
body: <p>I need to do something like...
  <code>
  "echo  --zones ",
       {
           "Ref": "az_of_subnet"
       },
  " &gt;&gt; aws_details.txt\n"</code></p>
  
  <p>in the user-data of an EC2 instance which is being created through the cloudformation script. The Subnet ID of an EXISTING subnet is known. Is there a way to get the availability zone of the subnet? Currently I'm taking this as an input (az_of_subnet) from the user, which seems redundant/not right, since I already know the subnet-id.</p>