dockerio: do not override default CMD declared in image

I'm using the following SLS to bootstrap https://github.com/shipyard/shipyard, as you can see I had to add `command: null` to force creating a container that uses the command declared as default by the docker image.

It doesn't make sense to me, so this pull request proposes changing salt's dockerio state to follow the defaults that comes from docker images as much as possible.

```
include:                                                                                                                                                                                                                            
  - docker                                                                                                                                                                                                                          
                                                                                                                                                                                                                                    
image:                                                                                                                                                                                                                              
  docker.pulled:                                                                                                                                                                                                                    
    - name: shipyard/shipyard                                                                                                                                                                                                       
    - require:                                                                                                                                                                                                                      
      - sls: docker                                                                                                                                                                                                                 
                                                                                                                                                                                                                                    
container:                                                                                                                                                                                                                          
  docker.installed:                                                                                                                                                                                                                 
    - name: shipyard                                                                                                                                                                                                                
    - image: shipyard/shipyard                                                                                                                                                                                                      
    - command: null                                                                                                                                                                                                                 
    - ports:                                                                                                                                                                                                                        
      - "80/tcp"                                                                                                                                                                                                                    
      - "8000/tcp"                                                                                                                                                                                                                  
    - environment:                                                                                                                                                                                                                  
      - CELERY_WORKERS: 3                                                                                                                                                                                                           
      - HIPACHE_WORKERS: 3                                                                                                                                                                                                          
    - require:                                                                                                                                                                                                                      
      - docker: image                                                                                                                                                                                                               
                                                                                                                                                                                                                                    
app:                                                                                                                                                                                                                                
  docker.running:                                                                                                                                                                                                                   
    - container: shipyard                                                                                                                                                                                                           
    - port_bindings:                                                                                                                                                                                                                
        "80/tcp":                                                                                                                                                                                                                   
            HostIp: ""                                                                                                                                                                                                              
            HostPort: "80"                                                                                                                                                                                                          
        "8000/tcp":                                                                                                                                                                                                                 
            HostIp: ""                                                                                                                                                                                                              
            HostPort: "8000"                                                                                                                                                                                                        
    - require:                                                                                                                                                                                                                      
      - docker: container 
```