[GCE] detect-master and detect-minions api calls should specify the zone

Otherwise KUBE_MASTER_IP and KUBE_MINION_IP_ADDRESSES may contain 'external-ip'

    $ detect-master
    Using master: kubernetes-master (external IP: external-ip)'
Merge pull request #1188 from carlossg/detect-zone

[GCE] detect-master and detect-minions api calls should specify the zone