[RAT-170] RAT should use itself during build and site generation
- Add a Rat report to the generated site. Had to move the configuration for Rat plugin to pluginManagement so that it can be shared under both build and reporting.

git-svn-id: https://svn.apache.org/repos/asf/creadur/rat/trunk@1622920 13f79535-47bb-0310-9956-ffa450edef68