Added ability to execute recipes using layer_name

I'm not sure which name you'd prefer to use to identify the layer, the name or the short-name. I picked name, but I kind of think short name might be better due to it's lack of spaces. 
Two things:

* You technically don't need the `layer_info` variable with how you have this code laid out because it isn't doing anything.
* Line 26 is pretty long and you should keep `layer_info` and do the line something like this:
  
  ```ruby
  layer_info = client.api_call('describe_layers', stack_id: @client.config.opsworks_config[:stack_id])
  layer_info[:layers].map do |layer|
    new(client, id: layer[:layer_id], name: layer[:shortname])
  end
  ```
  
  Arguably, you could make `layer_info` a method if you wanted.