HeliosClient should return JobDeleteResponse, not null, when deleting nonexistent job

Currently if you try to delete a nonexistent job, HeliosClient returns null,
so the caller doesn't know what happened. This caused a NPE in TemporaryJobs,
because it tried to delete a nonexistent job, and didn't expect the client to
return null. It now returns JobDeleteResponse with the status set to
JOB_NOT_FOUND so the caller can decide if this should be handled as an error
or not.
Merge pull request #103 from spotify/rculbertson/job-delete-response

HeliosClient should not return null, when deleting nonexistent job
Workaround for btrfs issue with removing containers

When using btrfs in certain environments (like CircleCI), the btrfs driver
throws an exception when attempting to remove a container. However, the
container does actually get removed.

Check for this specific case in our tests. This should only affect
AgentRestartTest, since it is the only thing that actually attempts to
remove a container.