docker: fix using images without a namespace

I wasn't able to run images without a namespace like `busybox`, I always got:

```
Traceback (most recent call last):
  File "/home/mantiz/.ansible/tmp/ansible-tmp-1410827919.81-129122313988220/docker", line 2086, in <module>
    main()
  File "/home/mantiz/.ansible/tmp/ansible-tmp-1410827919.81-129122313988220/docker", line 775, in main
    running_containers = manager.get_running_containers()
  File "/home/mantiz/.ansible/tmp/ansible-tmp-1410827919.81-129122313988220/docker", line 597, in get_running_containers
    for i in self.get_deployed_containers():
  File "/home/mantiz/.ansible/tmp/ansible-tmp-1410827919.81-129122313988220/docker", line 573, in get_deployed_containers
    image, tag = self.get_split_image_tag(image)
  File "/home/mantiz/.ansible/tmp/ansible-tmp-1410827919.81-129122313988220/docker", line 524, in get_split_image_tag
    registry, resource = image.rsplit('/', 1)
ValueError: need more than 1 value to unpack
```

I think this was introduced by 1c3039f0 where the check for `/` was removed.

I hope I didn't break anything.