agent: fix supervisor container teardown

When checking for if a container is no longer running, appropriately
deal with ContainerNotFoundException and interpret it as the container
no longer running (obviously, as it no longer even exists).

Saw the helios agent hitting this and logging stack traces for the
ContainerNotFoundException when using helios-testing and TemporaryJobs.
Merge pull request #166 from spotify/dano/agent-stop-kill-container-fix

agent: fix supervisor container teardown