Add hook script capability on droplet start/stop.

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)
  - ...

Change-Id: Ic4afc57a8d47b0797b93ce13c108d40a467dd091

(Moved from Gerrit: http://reviews.cloudfoundry.org/#/c/13516)