Pass a zuul scratch subpath to create-cgitrepos

* modules/openstack_project/manifests/git_backend.pp: Pass "zuul" as
the SCRATCH_SUBPATH environment variable for the create-cgitrepos
utility. With a new enough jeepyb to support this, it will generate
empty shadow git repositories where zuul can stow its throwaway job
references.

Change-Id: I26828ab9935e47382f6706a5c590f32b806d1713