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
Change 'stop' hooks timing since 'after_stop' does not make after warden container stopped...

The correct order is:

  RUNNING

  -> before_stop

  STOPPING

  -> after_stop

  STOPPED

Change-Id: I8aa3117e17ecb6da6bde412e83864f8ee366d9f1
added stop script test and refactoring of tests for script hooks.

Change-Id: If27105ccd50be09ba13580362588c11fbcf55d7f
Merge pull request #3 from yssk22/request/script-hooks

Add hook script capability on droplet start/stop.