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

Done. Thanks!