Allow zuul to push to git servers

* manifests/site.pp
* modules/openstack_project/manifests/git_backend.pp: Add the zuul
server's public SSH key to a dedicated zuul account on the git
server farm. This will allow zuul to push arbitrary references into
its scratch repositories. Also set ownership on its scratch
repository hierarchy and pack them, properly skipping it in the
existing cgit repo repack since ownership differs.

Change-Id: I4b5b583847c828aadc903270433882531e5b1ea7