Add .gitreview file.

Change-Id: I61f7a48d4a24885bbe271baab8398eb9accf3b6a
Reviewed-on: https://review.openstack.org/30116
Approved: Thierry Carrez <thierry@openstack.org>
Reviewed-by: Thierry Carrez <thierry@openstack.org>
Tested-by: Jenkins