cluster/gce: fix shell return value comparison.

In shell scripts inside [[ .. ]] blocks, ">" is a string comparison operator.
The "attempt" number comparison works (most likely by accident) because the max
number of attempts is below 10.  Change to -gt operator.

cluster/gce/list-resources.sh

@@ -57,7 +57,7 @@ function gcloud-compute-list() {
     fi
     echo -e "Attempt ${attempt} failed to list ${resource}. Retrying." >&2
     attempt=$(($attempt+1))
-    if [[ ${attempt} > 5 ]]; then
+    if [[ ${attempt} -gt 5 ]]; then
       echo -e "List ${resource} failed!" >&2
       exit 2
     fi