Rework cluster/ kube-proxy setup for --proxy-mode

cluster/gce/gci/configure-helper.sh

@@ -1753,7 +1753,12 @@ function prepare-kube-proxy-manifest-variables {
   if [[ -n "${FEATURE_GATES:-}" ]]; then
     params+=" --feature-gates=${FEATURE_GATES}"
   fi
-  if [[ "${KUBE_PROXY_MODE:-}" == "ipvs" ]];then
+
+  case "${KUBE_PROXY_MODE:-iptables}" in
+    iptables)
+      params+=" --proxy-mode=iptables --iptables-sync-period=1m --iptables-min-sync-period=10s"
+      ;;
+    ipvs)
       # use 'nf_conntrack' instead of 'nf_conntrack_ipv4' for linux kernel >= 4.19
       # https://github.com/kubernetes/kubernetes/pull/70398
       local -r kernel_version=$(uname -r | cut -d\. -f1,2)
@@ -1762,15 +1767,15 @@ function prepare-kube-proxy-manifest-variables {
         conntrack_module="nf_conntrack_ipv4"
       fi
 
-    if sudo modprobe -a ip_vs ip_vs_rr ip_vs_wrr ip_vs_sh ${conntrack_module}; then
-      params+=" --proxy-mode=ipvs"
-    else
+      if ! sudo modprobe -a ip_vs ip_vs_rr ip_vs_wrr ip_vs_sh ${conntrack_module}; then
         # If IPVS modules are not present, make sure the node does not come up as
         # healthy.
         exit 1
       fi
-  fi
-  params+=" --iptables-sync-period=1m --iptables-min-sync-period=10s --ipvs-sync-period=1m --ipvs-min-sync-period=10s"
+      params+=" --proxy-mode=ipvs --ipvs-sync-period=1m --ipvs-min-sync-period=10s"
+      ;;
+  esac
+
   if [[ -n "${KUBEPROXY_TEST_ARGS:-}" ]]; then
     params+=" ${KUBEPROXY_TEST_ARGS}"
   fi