[ingress] Support nginx-ingress-controller 0.30.0

In 0.30.0 (busybox inside) the "find" tool doesn't support
"writable" option, so use "perm" instead. Also get rid of
several system calls by means of make all by one command.

Change-Id: Ia4f7bc01fb61f4f32c21c50d8c4e870d0244c868

ingress/templates/bin/_ingress-controller.sh.tpl

@@ -18,7 +18,7 @@ set -ex
 COMMAND="${@:-start}"
 
 function start () {
-  find /tmp/ -maxdepth 1 -writable | grep -v "^/tmp/$" | xargs -L1 -r rm -rfv
+  find /tmp -maxdepth 1 \! -path /tmp -perm /222 -exec rm -rfv {} \;
 
   declare -A desired_opts
   desired_opts["--stream-port"]="${PORT_STREAM}"