Change the way we run e2e tests with feature gates in pipelines

Prior to this commit pipelines ran e2e tests for both "stable" and
"alpha" feature gates in a single prow job. This makes the tests
take twice as long as running running either individually.

In a parallel commit to Plumbing we are introducing a new prow job
that is specifically for running alpha e2e tests. This means both
the alpha and stable tests can be run concurrently and hopefully
should reduce the iteration time on PRs - failures should show up
sooner.

The alpha feature gate prow job will set the PIPELINE_FEATURE_GATE
env var when it is initiated. Our e2e script recognizes this var
and sets the feature-flags configmap accordingly. If the env var
is not set then we default to running under "stable" feature gate.

test/e2e-tests.sh

@@ -52,11 +52,14 @@ function run_e2e() {
   go_test_e2e -tags=examples -timeout=20m ./test/ || failed=1
 }
 
-set_feature_gate "stable"
-run_e2e
+if [ "$PIPELINE_FEATURE_GATE" == "" ]; then
+  set_feature_gate "stable"
+  run_e2e
+else
+  set_feature_gate "$PIPELINE_FEATURE_GATE"
+  run_e2e
+fi
 
-set_feature_gate "alpha"
-run_e2e
 
 (( failed )) && fail_test
 success