serviced snapshot commit will allow the short form of the container ID

DEMO1 - do no harm, it should work like it used to:
```
zenny@ip-10-111-3-51:~$ ./serviced service shell -i -s baz zope bash
I0927 05:30:34.707779 08243 server.go:341] Connected to the control center at port 10.111.3.51:4979
I0927 05:30:34.935757 08243 server.go:435] Acquiring image from the dfs...
I0927 05:30:34.936040 08243 server.go:437] Acquired!  Starting shell
Trying to connect to logstash server... 127.0.0.1:5042
Connected to logstash server.
bash-4.2# uname -n
c31cdc06ef34
bash-4.2# exit

zenny@ip-10-111-3-51:~$ docker ps --no-trunc -a |grep c31cdc06ef34
c31cdc06ef34795b867561d884c37588a2ca2c709dc9db8304511a984fa48a6e   ip-10-111-3-51:5000/2vlacjlun05czwlm1y8tqlcbk/resmgr-unstable:20140927-052815   "/serviced/serviced --logtostderr=false service proxy --autorestart=false --disable-metric-forwarding --logstash=false --logstash-idle-flush-time=0 --logstash-settle-time=0 esr59hlrvromn9cxtdu408hw4 0 bash"   26 seconds ago      Exited (0) 9 seconds ago                                                                                                    baz                                                           
zenny@ip-10-111-3-51:~$ ./serviced snapshot commit c31cdc06ef34795b867561d884c37588a2ca2c709dc9db8304511a984fa48a6e
2vlacjlun05czwlm1y8tqlcbk_20140927-053137
zenny@ip-10-111-3-51:~$ 
```

DEMO2 - allow short docker id:
```
zenny@ip-10-111-3-51:~$ ./serviced service shell -i -s foo zope bash
I0927 05:26:51.706751 09329 server.go:341] Connected to the control center at port 10.111.3.51:4979
I0927 05:26:51.932956 09329 server.go:435] Acquiring image from the dfs...
I0927 05:26:51.933217 09329 server.go:437] Acquired!  Starting shell
Trying to connect to logstash server... 127.0.0.1:5042
Connected to logstash server.
bash-4.2# uname -n
f7950f9adbd0
bash-4.2# exit

zenny@ip-10-111-3-51:~$ ./serviced snapshot commit f7950f9adbd0
2vlacjlun05czwlm1y8tqlcbk_20140927-052815
zenny@ip-10-111-3-51:~$ 
```