Fix GCE tests with undefined CI_PLATFORM (#4650)

.gitlab-ci/gce.yml

@@ -59,7 +59,7 @@ gce_centos7-flannel-addons:
 
 gce_centos-weave-kubeadm-sep:
   stage: deploy-gce
-  <<: *gce
+  extends: .gce
   variables:
     <<: *centos_weave_kubeadm_variables
   when: on_success
@@ -126,7 +126,7 @@ gce_ubuntu-flannel-ha:
 
 gce_centos-weave-kubeadm-triggers:
   stage: deploy-gce
-  <<: *gce
+  extends: .gce
   variables:
     <<: *centos_weave_kubeadm_variables
   when: on_success
@@ -227,7 +227,7 @@ gce_centos7-kube-router:
 
 gce_centos7-multus-calico:
   stage: deploy-gce
-  <<: *gce
+  extends: .gce
   variables:
     <<: *centos7_multus_calico_variables
   when: manual