Set ZUUL_REF for devstack plugins list proposal periodic job
Set ZUUL_REF for devstack plugins list proposal periodic job

Though branch-git-prep handles ZUUL_REF not being set, propose_update.sh
does not.  We thus set ZUUL_REF to master.

Change-Id: Ie2dd25e312d0a7f56bc8b7eac27cfe0c41b9041a

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