Docker: Handle images with multiple tags

Scenario: Tags returned from docker and ansible may not always match
---

Single docker image has multiple tags associated (latest, prod, 456). Playbook looks like this:

```
tasks:
    - name: stop old containers
      local_action:
        module: docker
        docker_url: tcp://{{ facter_ipaddress_eth0 }}:2375
        image: hub.example.com/base
        state: stopped
    - name: run container
      local_action:
        module: docker
        docker_url: tcp://{{ facter_ipaddress_eth0 }}:2375
        image: hub.example.com/base:prod
        command: sleep 300
        count: 3
        state: running
```

Before the patch you end up with 3 new containers and the old containers would still be running. Docker will return the tag 456 (or one of the other tags) and the match in docker.py will fail. The patch will get all tags associated with the given image and return the correct list of containers.

Cheers :beers: 
Daniel
Might be slightly better to use a set instead of a list here since we don't care about order and we're doing a "img in all_valid_tags" check later.