Don't use --skip-redirect for cinder restart on rhel6

RHEL6 doesn't support this flag so the restart fails.  Not exactly
sure why it is required, seems unchagned from the initial
commit 67787e6b4c

(found running stack.sh with -e per [1])

[1] https://review.openstack.org/#/c/71996/

Change-Id: Ib34c3663409d7b96b932286cb5a6974e940075d3

lib/cinder

@@ -496,8 +496,12 @@ function start_cinder() {
             sudo stop tgt || true
             sudo start tgt
         elif is_fedora; then
+            if [[ $DISTRO =~ (rhel6) ]]; then
+                sudo /sbin/service tgtd restart
+            else
                 # bypass redirection to systemctl during restart
                 sudo /sbin/service --skip-redirect tgtd restart
+            fi
         elif is_suse; then
             restart_service tgtd
         else