Fix workspace setup

Stable branch overrides for client testing, devstack-gate alternate-branch
testing, and grenade old/new branch selection are all actually the same
thing.  Update setup_project to reflect that, fixing all known bugs in
the process.  Note that some jobs will need to be updated to pass in
OVERRIDE_ZUUL_BRANCH.

Change-Id: I6613715439bed9914b8444e335fac5eab4d14075