Ensure clients get tested correctly in upgrades

Client libraries don't have branches, so when a change comes in
from python-novaclient the ZUUL_BRANCH is always master. That means
we need to actually check to see if ZUUL_BRANCH is BRANCH, not
base_branch, which is what we were passed by setup_workspace.

Change-Id: I6c7811b279d3450400bb5be140851dbfb52dbc16