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. 
So based on what @anfleene suggested, I would probably change my above snippet to this:

```
def determine_instance_ids(options)
  if options[:instance_ids]
    options[:instance_ids]
  elsif options[:layers]
    Layer.get_instance_ids_from_layers options[:layers]
  end
end
```