Configurable upstart start and stop commands

In working with the cookbook I encountered a problem where the etcd service would not restart when the VM I was testing in was restarted.  I tracked this to the way CentOS responds to the etcd.conf file for upstart.  Instead of using the 'started networking' and 'shutdown' values I was able to have CentOS restart the service by specifying runlevels

```
start on runlevel [2345]
stop on runlevel [06]
```

In order to avoid messy OS conditional logic I opted instead to make the values configurable via attributes with a sensible Ubuntu default since no other users had reported an issue.  Let me know if you'd like a different conditional solution and I'll give it a shot.