Add .gitreview config file for gerrit.
The CI team is developing a new tool, git-review: https://github.com/openstack-ci/git-review which is intendend to replace rfc.sh. This adds a .gitreview file so that it can automatically determine the canonical gerrit location for the repository when first run. Later, rfc.sh will be updated to indicate it is deprecated, and then eventually removed. Change-Id: I49edfa303671de58f54eae4e1d7c27916c41dda7changes/59/1059/2