Unset ZUUL_PROJECT on periodic devstack jobs

The Zuul periodic trigger sets it, but devstack-gate expects
that if it is set, then ZUUL_REF will also be set, which it is
not (yet).

Change-Id: Id5ad96fca83c7e8c50f567fa6220ad00f1252e41
Merge "Unset ZUUL_PROJECT on periodic devstack jobs"