Provide ZUUL_BRANCH in constraints generation job
Provide ZUUL_BRANCH in constraints generation job

The constraints generation job script uses zuul-cloner, so set
ZUUL_BRANCH since it's run from a periodic pipeline where Zuul
doesn't provide the ZUUL_BRANCH nor ZUUL_REF job parameters
zuul-cloner requires.

Change-Id: I65d3aa38eb45cfbecf13561d3af4b4e0235f2f5d

dongwenjuan/project-config,Tesora/tesora-project-config,anbangr/osci-project-config,coolsvap/project-config,anbangr/osci-project-config,noorul/os-project-config,noorul/os-project-config,coolsvap/project-config,openstack-infra/project-config,dongwenjuan/project-config,Tesora/tesora-project-config,openstack-infra/project-config