e2e: Add container runtime flag

hack/ginkgo-e2e.sh

@@ -119,6 +119,7 @@ export PATH=$(dirname "${e2e_test}"):"${PATH}"
   --node-instance-group="${NODE_INSTANCE_GROUP:-}" \
   --prefix="${KUBE_GCE_INSTANCE_PREFIX:-e2e}" \
   ${OS_DISTRIBUTION:+"--os-distro=${OS_DISTRIBUTION}"} \
+  ${KUBE_CONTAINER_RUNTIME:+"--container-runtime=${KUBE_CONTAINER_RUNTIME}"} \
   ${NUM_NODES:+"--num-nodes=${NUM_NODES}"} \
   ${E2E_CLEAN_START:+"--clean-start=true"} \
   ${E2E_MIN_STARTUP_PODS:+"--minStartupPods=${E2E_MIN_STARTUP_PODS}"} \

test/e2e/framework/test_context.go

@@ -47,6 +47,7 @@ type TestContextType struct {
 	UpgradeTarget            string
 	PrometheusPushGateway    string
 	OSDistro                 string
+	ContainerRuntime         string
 	VerifyServiceAccount     bool
 	DeleteNamespace          bool
 	CleanStart               bool
@@ -108,6 +109,7 @@ func RegisterFlags() {
 	flag.StringVar(&TestContext.ReportPrefix, "report-prefix", "", "Optional prefix for JUnit XML reports. Default is empty, which doesn't prepend anything to the default name.")
 	flag.StringVar(&TestContext.Prefix, "prefix", "e2e", "A prefix to be added to cloud resources created during testing.")
 	flag.StringVar(&TestContext.OSDistro, "os-distro", "debian", "The OS distribution of cluster VM instances (debian, trusty, or coreos).")
+	flag.StringVar(&TestContext.ContainerRuntime, "container-runtime", "docker", "The container runtime of cluster VM instances (docker or rkt).")
 
 	// TODO: Flags per provider?  Rename gce-project/gce-zone?
 	cloudConfig := &TestContext.CloudConfig