configure_instances playbook on GCE fails

[nicoaws]

following (ad litteram) the example at gce1.md, the configure instances playbook fails

TASK [Gathering Facts] ***********************************************************************************************************************************************************
fatal: [35.185.237.69]: UNREACHABLE! => {"changed": false, "msg": "Failed to connect to the host via ssh: [email protected]: Permission denied (publickey,gssapi-keyex,gssapi-with-mic).\r\n", "unreachable": true}
fatal: [35.233.220.159]: UNREACHABLE! => {"changed": false, "msg": "Failed to connect to the host via ssh: [email protected]: Permission denied (publickey,gssapi-keyex,gssapi-with-mic).\r\n", "unreachable": true}
fatal: [35.233.202.87]: UNREACHABLE! => {"changed": false, "msg": "Failed to connect to the host via ssh: [email protected]: Permission denied (publickey,gssapi-keyex,gssapi-with-mic).\r\n", "unreachable": true}
fatal: [35.230.33.116]: UNREACHABLE! => {"changed": false, "msg": "Failed to connect to the host via ssh: [email protected]: Permission denied (publickey,gssapi-keyex,gssapi-with-mic).\r\n", "unreachable": true}
fatal: [35.227.157.13]: UNREACHABLE! => {"changed": false, "msg": "Failed to connect to the host via ssh: [email protected]: Permission denied (publickey,gssapi-keyex,gssapi-with-mic).\r\n", "unreachable": true}
fatal: [35.199.160.152]: UNREACHABLE! => {"changed": false, "msg": "Failed to connect to the host via ssh: [email protected]: Permission denied (publickey,gssapi-keyex,gssapi-with-mic).\r\n", "unreachable": true}

provider_config:
  gce:
    service_account_email: [omitted]
    credentials_file: [omitted]/projects/contrail-ansible-deployer/vigilant-design-134823-6eb1d1d5e0eb.json
    project_id: [omitted]
    ssh_user: centos
    machine_type: n1-standard-4
    image: centos-7
    network: microservice-vn
    subnetwork: microservice-sn
    zone: us-west1-a
    disk_size: 50
instances:
  master1:
    provider: gce
    roles:
      config_database:
      config:
      control:
      analytics_database:
      analytics:
      webui:
      k8s_master:
  master2:
    provider: gce
    roles:
      config_database:
      config:
      control:
      analytics_database:
      analytics:
      webui:
      k8s_master:
  master3:
    provider: gce
    roles:
      config_database:
      config:
      control:
      analytics_database:
      analytics:
      webui:
      k8s_master:
  node1:
    provider: gce
    roles:
      vrouter:
      k8s_node:
  node2:
    provider: gce
    roles:
      vrouter:
      k8s_node:
  node3:
    provider: gce
    roles:
      vrouter:
      k8s_node:

[nicoaws]

BTW, ssh with private key works fine!