cluster/gce: fix checks for empty strings.

In order to use -n, the value needs either be quoted or [[ .. ]] block
has to be used. Fix the comparisons that way.

To verify, consider this (analogous) script:

  #!/bin/bash

  subnetwork_url=""

  if [ -n ${subnetwork_url} ]; then
    echo "foo"
  fi

  if [[ -n ${subnetwork_url} ]]; then
    echo "bar"
  fi

Here "foo" is echoed by the script, even though the variable
subnetwork_url has a zero-length value.

cluster/gce/gci/flexvolume_node_setup.sh

@@ -81,10 +81,10 @@ flex_clean() {
   umount_silent ${MOUNTER_PATH}
   rm -rf ${MOUNTER_PATH}
   
-  if [ -n ${IMAGE_URL:-} ]; then
+  if [[ -n ${IMAGE_URL:-} ]]; then
     docker rmi -f ${IMAGE_URL} &> /dev/null || /bin/true
   fi
-  if [ -n ${MOUNTER_DEFAULT_NAME:-} ]; then
+  if [[ -n ${MOUNTER_DEFAULT_NAME:-} ]]; then
     docker rm -f ${MOUNTER_DEFAULT_NAME} &> /dev/null || /bin/true
   fi
 }

cluster/gce/upgrade-aliases.sh

@@ -53,7 +53,7 @@ function detect-k8s-subnetwork() {
   local subnetwork_url=$(gcloud compute instances describe \
     ${KUBE_MASTER} --project=${PROJECT} --zone=${ZONE} \
     --format='value(networkInterfaces[0].subnetwork)')
-  if [ -n ${subnetwork_url} ]; then
+  if [[ -n ${subnetwork_url} ]]; then
     IP_ALIAS_SUBNETWORK=$(echo ${subnetwork_url##*/})
   fi
 }