Standardize on KUBE_PROXY_MODE (not KUBEPROXY_MODE)

There's a disconnect between 12d4eac038
and the earlier a6af827898. One says
KUBEPROXY_MODE and another says KUBE_PROXY_MODE. Let's just pick one.
Currently we fail with the following error:

hack/local-up-cluster.sh: line 808: KUBEPROXY_MODE: unbound variable

hack/local-up-cluster.sh

@@ -121,7 +121,7 @@ if [ "${CLOUD_PROVIDER}" == "openstack" ]; then
 fi
 
 # set feature gates if using ipvs mode
-if [ "${KUBEPROXY_MODE}" == "ipvs" ]; then
+if [ "${KUBE_PROXY_MODE}" == "ipvs" ]; then
     FEATURE_GATES="$FEATURE_GATES,SupportIPVSProxyMode=true"
 fi
 
@@ -811,9 +811,9 @@ clientConnection:
   kubeconfig: ${CERT_DIR}/kube-proxy.kubeconfig
 hostnameOverride: ${HOSTNAME_OVERRIDE}
 featureGates: ${FEATURE_GATES}
-mode: ${KUBEPROXY_MODE}
+mode: ${KUBE_PROXY_MODE}
 EOF
-    if [ "${KUBEPROXY_MODE}" == "ipvs" ]; then
+    if [ "${KUBE_PROXY_MODE}" == "ipvs" ]; then
 	# Load kernel modules required by IPVS proxier
         sudo modprobe -a ip_vs ip_vs_rr ip_vs_wrr ip_vs_sh nf_conntrack_ipv4
     fi