Plugin LWRP & Other Fun Things

- Work around rare bug when updating metadata (bug)
- Use ec2.userdata automatically provided by Ohai (speed)
- Move agent config to `/etc/sd-agent/conf.d/agent.cfg` (improvement)
- Use single quotes when not interpolating (style)
- Provide sd-agent service resource for external notifications (feature)
- Plugin support (feature)
- Write up plugin LWRP (docs)