Allow make-ca-cert.sh to run on OS X

Currently make-ca-cert.sh uses (equiv of)

mktemp -d --tmpdir kube.XXXXX

but --tmpdir is not a valid option on OS X. Switch to

mktemp -d -t kube.XXXXX

Which is valid, but subtly different between OS X and Linux. The
directory you get back will be different on each.

Linux:  ${tmpdir}/kube.y5Bsu/
OS X:   ${tmpdir}/kube.XXXXX.VQ81oOui/

cluster/saltbase/salt/generate-cert/make-ca-cert.sh

@@ -46,7 +46,7 @@ if [[ -n "${extra_sans}" ]]; then
   sans="${sans},${extra_sans}"
 fi
 
-tmpdir=$(mktemp -d --tmpdir kubernetes_cacert.XXXXXX)
+tmpdir=$(mktemp -d -t kubernetes_cacert.XXXXXX)
 trap 'rm -rf "${tmpdir}"' EXIT
 cd "${tmpdir}"