tests: Prepull images commonly used test images

Some tests have a short timeout for starting the pods (1 minute), but if
those tests happen to be the first ones to run, and the images have to be
pulled, then the test could timeout, especially with larger images. This
commit will allow us to prepull commonly used E2E test images, so this issue
can be avoided.

hack/ginkgo-e2e.sh

@@ -189,6 +189,7 @@ fi
   --master-tag="${MASTER_TAG:-}" \
   --docker-config-file="${DOCKER_CONFIG_FILE:-}" \
   --dns-domain="${KUBE_DNS_DOMAIN:-cluster.local}" \
+  --prepull-images="${PREPULL_IMAGES:-false}" \
   --ginkgo.slowSpecThreshold="${GINKGO_SLOW_SPEC_THRESHOLD:-300}" \
   ${CONTAINER_RUNTIME:+"--container-runtime=${CONTAINER_RUNTIME}"} \
   ${MASTER_OS_DISTRIBUTION:+"--master-os-distro=${MASTER_OS_DISTRIBUTION}"} \