fix tgt to use 'service' instead of upstart calls

the comments in here were largely about oneric, which we don't
support any more. service is installed in a precise environment,
and will support debian and the upcoming transition to systemd
better, so use that instead.

Change-Id: If15493549a8c93a7387df9b3bba31443aed46995

lib/cinder

@@ -491,10 +491,7 @@ function start_cinder {
         sudo rm -f /etc/tgt/conf.d/stack.conf
         _configure_tgt_for_config_d
         if is_ubuntu; then
-            # tgt in oneiric doesn't restart properly if tgtd isn't running
-            # do it in two steps
-            sudo stop tgt || true
-            sudo start tgt
+            sudo service tgt restart
         elif is_fedora; then
             if [[ $DISTRO =~ (rhel6) ]]; then
                 sudo /sbin/service tgtd restart