fix grenade branch selection

the grenade branch selection ignored the ability to do branch
override in jobs, as such wasn't correctly enforcing things on
branchless tempest. Use OVERRIDE_ZUUL_BRANCH earlier in grenade
branch selection to prevent us from landing tempest tests that
will break grenade upgrades on stable/icehouse.

Change-Id: I55fa11f685fb33580835ffbad34eb7fb140f7e75