This change adds a DevStack plugin for Group-Based Policy. A new top-level directory, devstack, has been created to fulfill the contract of the DevStack plugin mechanism [1]. Documentation on how to enable the plugin has been added to doc/source/installation.rst. [1] http://docs.openstack.org/developer/devstack/plugins.html Change-Id: I7e6c95408fbb132e33823d803a44f2867541a922 Closes-Bug: #1564893