Prevent Upstart post-start stanza from hanging

Once the job has failed and is respawned, the status becomes `docker
respawn/post-start` after subsequent failures (as opposed to `docker
stop/post-start`), so the post-start script needs to take this into
account.

I could not find specific documentation on the job transitioning to the
`respawn/post-start` state, but this was observed on Ubuntu 14.04.2.

Signed-off-by: Lewis Marshall <lewis@lmars.net>