Sketch of implementation.
A NamespacedDockerClient that isn't actually namespaced yet, but is a
docker client.
Add a container_name attribute to Unit.

This allows some improvements to tests since we now know where to find
the container.
Delete some unused imports.
NamespacedDockerClient now works.
Skip tests if Docker is unavailable.
Fix some other tests that used Unit().
Lint fix.
Address review comment: Include test name in namespace.
Merge pull request #841 from ClusterHQ/dockerclient-namespacing-772

Dockerclient namespacing.

Fixes #772.