Adding optional shutdown hook

Based on the blog post [here](http://copperegg.com/hooking-into-the-aws-shutdown-flow/)

Added `default[:copperegg][:shutdown_hook]` attribute
Added `shutdown.sh.erb` template

Left out the removal of the probe data in `shutdown.sh` as that can still be managed with the probe LWRP.


NOTE: This was only tested on Ubuntu 13.04 but don't see why it wouldn't work for other OSs.