hack/tf-fmt: Allow callers to pass positional args

This way they can use:

  $ hack/tf-fmt.sh -list -check

to update their local Terraform (vs. just checking if it's correctly
formatted).  I've also dropped the read-only mount option to support
this use case.

I use set [1] to inject default args if the caller set no positional
args themselves [2], so the test config in openshift/release continues
to work.

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_05_02

hack/tf-fmt.sh

@@ -2,12 +2,15 @@
 
 # in prow, already in container, so no 'podman run'
 if [ "$IS_CONTAINER" != "" ]; then
+  if [ "${#N}" -gt 1 ]; then
+    set -- -list -check -write=false
+  fi
   set -x
-  /terraform fmt -list -check -write=false
+  /terraform fmt "${@}"
 else
   podman run --rm \
     --env IS_CONTAINER=TRUE \
-    --volume "${PWD}:${PWD}:ro,z" \
+    --volume "${PWD}:${PWD}:z" \
     --workdir "${PWD}" \
     quay.io/coreos/terraform-alpine:v0.11.7 \
     ./hack/tf-fmt.sh