Delete network by default in kube-down unless using default network

cluster/gce/config-default.sh

@@ -43,7 +43,7 @@ REGISTER_MASTER_KUBELET=${REGISTER_MASTER:-true}
 PREEMPTIBLE_NODE=${PREEMPTIBLE_NODE:-false}
 PREEMPTIBLE_MASTER=${PREEMPTIBLE_MASTER:-false}
 KUBE_DELETE_NODES=${KUBE_DELETE_NODES:-true}
-KUBE_DELETE_NETWORK=${KUBE_DELETE_NETWORK:-false}
+KUBE_DELETE_NETWORK=${KUBE_DELETE_NETWORK:-} # default value calculated below
 CREATE_CUSTOM_NETWORK=${CREATE_CUSTOM_NETWORK:-false}
 
 MASTER_OS_DISTRIBUTION=${KUBE_MASTER_OS_DISTRIBUTION:-${KUBE_OS_DISTRIBUTION:-gci}}
@@ -91,6 +91,12 @@ MASTER_EXTRA_METADATA=${KUBE_MASTER_EXTRA_METADATA:-${KUBE_EXTRA_METADATA:-}}
 NODE_EXTRA_METADATA=${KUBE_NODE_EXTRA_METADATA:-${KUBE_EXTRA_METADATA:-}}
 
 NETWORK=${KUBE_GCE_NETWORK:-default}
+# Enable network deletion by default (for kube-down), unless we're using 'default' network.
+if [[ "${NETWORK}" == "default" ]]; then
+  KUBE_DELETE_NETWORK=${KUBE_DELETE_NETWORK:-false}
+else
+  KUBE_DELETE_NETWORK=${KUBE_DELETE_NETWORK:-true}
+fi
 if [[ "${CREATE_CUSTOM_NETWORK}" == true ]]; then
   SUBNETWORK="${SUBNETWORK:-${NETWORK}-custom-subnet}"
 fi