Only zuulify the tree if we are on the right branch

We need to be more careful in our selection of when we zuulify the
tree, because we now want to zuulify the "old" tree as well. But
only do that if the ZUUL_BRANCH matches the $branch we've told it
to setup the workspace for.

Change-Id: Ie0ef097949aa54cb720615df80ed3aba4c16cc30