Quote potentially empty vars.

This was causing an error if $PROJECT was empty (eg, periodic jobs).

Change-Id: Ia05a8a0166d356491217dbf6e00938c4586c80ab

devstack-vm-gate-wrap.sh

@@ -130,7 +130,7 @@ function setup_workspace {
         git reset --hard FETCH_HEAD
         git clean -x -f -d -q
       else
-        if [ $PROJECT == $ZUUL_PROJECT ]; then
+        if [ "$PROJECT" == "$ZUUL_PROJECT" ]; then
           echo "Unable to find ref $ZUUL_REF for $PROJECT"
           exit 1
         fi