[FLOC-1045] Handle missing containers in DockerClient.list

Check for 404 exceptions when we call `inspect_container` in `DockerClient.list` because the container may have been removed by another thread.

Fixes https://clusterhq.atlassian.net/browse/FLOC-1045
Rather than using a random name, why not put it in `NamespacedDockerClientTests` and use the generated namespace?