Using containervm image for gce by default

Have done a lot of tests against this change: 

1) Verified with kube-up.sh, and both docker and kubernetes binary are upgraded properly:

On master node: 
root@kubernetes-master:/var/log# /usr/local/bin/apiserver --version
Kubernetes 3f52a97052dd1b2880e2fae0e41338f852d32a41-311-gdcd454374fe4e1-dirty

On minion node:
root@kubernetes-minion-1:/var/log# /usr/local/bin/kubelet --version
Kubernetes 3f52a97052dd1b2880e2fae0e41338f852d32a41-311-gdcd454374fe4e1-dirty
root@kubernetes-minion-1:/var/log# /usr/bin/docker --version
Docker version 1.3.0, build c78088f

2) With #2042 merged, e2e tests are passed:
2014/10/28 15:29:39 Passed tests: [update.sh guestbook.sh services.sh goe2e.sh basic.sh]
2014/10/28 15:29:39 Failed tests: []
+ exit 0

3) ran replication controller example by following examples/guestbook:

$ cluster/kubecfg.sh list pods
Name                                   Image(s)                   Host                                                               Labels              Status
----------                             ----------                 ----------                                                         ----------          ----------
redis-master-2                         dockerfile/redis           kubernetes-minion-1.c.golden-system-455.internal/146.148.52.185    name=redis-master   Running
12ca8706-5efe-11e4-b6bd-42010af0091c   brendanburns/redis-slave   kubernetes-minion-4.c.golden-system-455.internal/130.211.121.128   name=redisslave     Running
12cbf5f5-5efe-11e4-b6bd-42010af0091c   brendanburns/redis-slave   kubernetes-minion-2.c.golden-system-455.internal/107.178.223.69    name=redisslave     Running
41141971-5efe-11e4-b6bd-42010af0091c   brendanburns/php-redis     kubernetes-minion-3.c.golden-system-455.internal/130.211.119.51    name=frontend       Running
41144698-5efe-11e4-b6bd-42010af0091c   brendanburns/php-redis     kubernetes-minion-1.c.golden-system-455.internal/146.148.52.185    name=frontend       Running
41149190-5efe-11e4-b6bd-42010af0091c   brendanburns/php-redis     kubernetes-minion-4.c.golden-system-455.internal/130.211.121.128   name=frontend       Running

Yes. the latest gcloud SDK release (or candidate) finally has  containervm image as another official image provided by gce. But even that, we might want to explicitly control which release of containervm image to use for now.