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: Ifc1d34143b1b90faa4324f7750bfc43f69a5f494

Author: jeblair

.gitreview

@@ -0,0 +1,4 @@
+[gerrit]
+host=review.openstack.org
+port=29418
+project=openstack/nova.git