Fix nova volume install for devstack

This changes does mkdir -p /etc/tgt/conf.d to avoid the following
error, when nova volume is enabled (perhaps limited to xenapi)
tee: /etc/tgt/conf.d/nova.conf: No such file or directory

Change-Id: Ia5804c7ff50fbf1bc41e14ad13923c9261b000e0
diff --git a/stack.sh b/stack.sh
index b091fe3..c7d83f5 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1746,6 +1746,7 @@
 
         # Setup the tgt configuration file
         if [[ ! -f /etc/tgt/conf.d/nova.conf ]]; then
+           sudo mkdir -p /etc/tgt/conf.d
            echo "include $NOVA_DIR/volumes/*" | sudo tee /etc/tgt/conf.d/nova.conf
         fi