make more of the shell pass lints

cluster/images/etcd-empty-dir-cleanup/etcd-empty-dir-cleanup.sh

@@ -17,14 +17,14 @@
 echo "Removing empty directories from etcd..."
 
 cleanup_empty_dirs () {
-  if [ "$(${ETCDCTL} ls $1)" ]; then
-    for SUBDIR in $(${ETCDCTL} ls -p $1 | grep "/$")
+  if [ "$("${ETCDCTL}" ls "${1}")" ]; then
+    for SUBDIR in $("${ETCDCTL}" ls -p "${1}" | grep "/$")
     do
-      cleanup_empty_dirs ${SUBDIR}
+      cleanup_empty_dirs "${SUBDIR}"
     done
   else
     echo "Removing empty key $1 ..."
-    ${ETCDCTL} rmdir $1
+    "${ETCDCTL}" rmdir "${1}"
   fi
 }
 
@@ -33,5 +33,5 @@ do
   echo "Starting cleanup..."
   cleanup_empty_dirs "/registry"
   echo "Done with cleanup."
-  sleep ${SLEEP_SECOND}
+  sleep "${SLEEP_SECOND}"
 done