buildscripts: Fix kube contexts in the xds LB tests (#9389)

- The primary should've been `GKE_CLUSTER_PSM_LB`
- The secondary cluster was not activated for LB tests. This resulted
  in the failover test failing, as it relies on workloads running in
  different zones.

buildscripts/kokoro/xds_k8s_lb.sh

@@ -102,6 +102,7 @@ build_docker_images_if_needed() {
 # Globals:
 #   TEST_DRIVER_FLAGFILE: Relative path to test driver flagfile
 #   KUBE_CONTEXT: The name of kubectl context with GKE cluster access
+#   SECONDARY_KUBE_CONTEXT: The name of kubectl context with secondary GKE cluster access, if any
 #   TEST_XML_OUTPUT_DIR: Output directory for the test xUnit XML report
 #   SERVER_IMAGE_NAME: Test server Docker image name
 #   CLIENT_IMAGE_NAME: Test client Docker image name
@@ -120,6 +121,7 @@ run_test() {
   python -m "tests.${test_name}" \
     --flagfile="${TEST_DRIVER_FLAGFILE}" \
     --kube_context="${KUBE_CONTEXT}" \
+    --secondary_kube_context="${SECONDARY_KUBE_CONTEXT}" \
     --server_image="${SERVER_IMAGE_NAME}:${GIT_COMMIT}" \
     --client_image="${CLIENT_IMAGE_NAME}:${GIT_COMMIT}" \
     --testing_version="${TESTING_VERSION}" \
@@ -156,7 +158,8 @@ main() {
   echo "Sourcing test driver install script from: ${TEST_DRIVER_INSTALL_SCRIPT_URL}"
   source /dev/stdin <<< "$(curl -s "${TEST_DRIVER_INSTALL_SCRIPT_URL}")"
 
-  activate_gke_cluster GKE_CLUSTER_PSM_BASIC
+  activate_gke_cluster GKE_CLUSTER_PSM_LB
+  activate_secondary_gke_cluster GKE_CLUSTER_PSM_LB
 
   set -x
   if [[ -n "${KOKORO_ARTIFACTS_DIR}" ]]; then