Add image type env var option for k8s integration script

Author: davidz627

test/run-k8s-integration.sh

@@ -19,6 +19,7 @@ readonly kube_version=${GCE_PD_KUBE_VERSION:-master}
 readonly test_version=${TEST_VERSION:-master}
 readonly gce_zone=${GCE_CLUSTER_ZONE:-us-central1-b}
 readonly gce_region=${GCE_CLUSTER_REGION:-}
+readonly image_type=${IMAGE_TYPE:-cos}
 
 export GCE_PD_VERBOSITY=9
 
@@ -28,7 +29,8 @@ base_cmd="${PKGDIR}/bin/k8s-integration-test \
             --run-in-prow=true --deploy-overlay-name=${overlay_name} --service-account-file=${E2E_GOOGLE_APPLICATION_CREDENTIALS} \
             --do-driver-build=${do_driver_build} --boskos-resource-type=${boskos_resource_type} \
             --storageclass-file=sc-standard.yaml --test-focus="External.Storage" \
-            --deployment-strategy=${deployment_strategy} --test-version=${test_version} --num-nodes=3"
+            --deployment-strategy=${deployment_strategy} --test-version=${test_version} --num-nodes=3" \
+            --image-type=${image_type}
 
 if [ "$deployment_strategy" = "gke" ]; then
   base_cmd="${base_cmd} --gke-cluster-version=${gke_cluster_version}"