Enable to e2e-test with contour and kourier

[BbolroC]

The current e2e test only runs over the network istio. It would be good to run the test over other network layers (e.g. contour and kourier) if the network is pre-installed and the environment variable $INGRESS_CLASS is set as follows:

diff --git a/test/common.sh b/test/common.sh
index 4d706168..e54a52ac 100755
--- a/test/common.sh
+++ b/test/common.sh
@@ -16,6 +16,12 @@
 
 source $(dirname $0)/../vendor/knative.dev/hack/e2e-tests.sh
 
+export INGRESS_CLASS=${INGRESS_CLASS:-istio.ingress.networking.knative.dev}
+
+function is_ingress_class() {
+  [[ "${INGRESS_CLASS}" == *"${1}"* ]]
+}
+
 function cluster_setup() {
   header "Building client"
   ${REPO_ROOT_DIR}/hack/build.sh -f || return 1
@@ -59,7 +65,9 @@ function install_istio() {
 }
 
 function knative_setup() {
-  install_istio
+  if is_ingress_class istio; then
+    install_istio
+  fi
 
   local serving_version=${KNATIVE_SERVING_VERSION:-latest}
   header "Installing Knative Serving (${serving_version})"
@@ -88,6 +88,11 @@ function knative_setup() {
     wait_until_pods_running knative-serving || return 1
   fi
 
+  if ! is_ingress_class istio; then
+    kubectl patch configmap/config-network -n knative-serving \
+      --type merge -p '{"data": {"ingress.class":"'${INGRESS_CLASS}'"}}'
+  fi
+
   local eventing_version=${KNATIVE_EVENTING_VERSION:-latest}
   header "Installing Knative Eventing (${eventing_version})"

I would like to get some feedback on this. 😉