[hopem,r=billy-olsen] Cleanup temp dir.

Ensure mktemp dir is cleaned up when no longer needed.

Closes-Bug: 1467938

templates/os-charm-phy-nic-mtu.conf

@@ -7,9 +7,9 @@ task
 script
     devs="{{ devs }}"
     mtu="{{ mtu }}"
-    tmpfile=`mktemp`
-    echo $devs > $tmpfile
     if [ -n "$mtu" ]; then
+        tmpfile=`mktemp`
+        echo $devs > $tmpfile
         while read -r dev; do
             [ -n "$dev" ] || continue
             rc=0
@@ -19,4 +19,4 @@ script
         rm $tmpfile
         [ $rc = 0 ] || exit $rc
     fi
-end script
+end script