Add hook script capability on droplet start/stop.

Retry https://github.com/cloudfoundry/dea_ng/pull/1

Hook script provide the CF providers to configure warden container based
on their demand. If providers want to start a special process for their purpose (*1)
in droplet container, they can setup it on container_root_fs (of Warden)
by chroot/apt-get combination and start the daemon by hook script.

(*1) e.g:

Log Collector daemon (flume, fluentd, ...etc),
Redis proxy daemon (Twemproxy to proxy requests to bound redis services)
...