Correct test for zuulifying the tree

The new zuulification of old requires that we know that we are on
the branch which corresponds to the old change. This means that
instead of having a flag to setup_workspace about whether we are
supposed to set it up, we instead need a check to ensure that it's
actually the ZUUL_BRANCH.

The previous attempt at this failed because the condition was too
nested because I didn't not fully understand the complexity of the
multilevel, multiply segmented, if condition that this was
attempted to be inserted into. By fully extracting it one level
this should be correct.

Change-Id: Ie787de3cb0b42b6b53b149dd528e0a20aae5c328