Dockerclient namespacing - fixes 772

Fixes #772.

More like an hour and a half, in the end.
It might be nice to set the namespace to the name of the TestCase. That'll allow us to identify which containers are not being cleaned up by which tests. For example, I set it to RICHARDW and found a left over container when the tests had finished.

```
(772)[vagrant@localhost vagrant]$ sudo docker ps -a
CONTAINER ID        IMAGE                       COMMAND             CREATED             STATUS                      PORTS               NAMES
192fe22bc616        busybox:buildroot-2014.02   "/bin/sh"           30 seconds ago      Exited (0) 30 seconds ago                       flocker--506488511904--291924082095   
f0c0ca3475c9        busybox:buildroot-2014.02   "/bin/sh"           33 seconds ago      Exited (0) 32 seconds ago                       flocker--RICHARDW--3718519865     
```