A test to demonstrate the race condition in DockerClient.list
Simplify
Move test to existing TestCase and check the returned list of units.
Check for 404 when inspecting the returned list
This test won't work with NamespacedDockerClient so move it to the DockerClient specific tests
Add a namespace to avoid listing containers from other tests
Flake8
Use the namespace generation we already have, rather than a random named namespace.
Don't say flocker_docker_client._client 3 times.
Actually factor out the namespace generation.
Merge pull request #1003 from ClusterHQ/dockerclient-list-race-FLOC-1045

[FLOC-1045] Handle missing containers in DockerClient.list.

Author: @walljr
Reviewer: @tomprince