Currently the incremental push of the git repos to the nodes is split
into a part in project-config and a part in zuul-jobs and only works
within the OpenStack context. The prepare-workspace-git role is an
attempt to make this generically available in zuul-jobs.
It first checks for each needed project if it's cached. If yes it's
cloned to the destination, if not it's just initialized as an empty
git repo. After that we can generically use git operations to sync the
correct state of the repos to the destination.
Change-Id: I87938a02d51e561b25ca2cb20f53d62f3cd3ae73
7cee7156bc