[MCHECKSTYLE-172] Checkstyle Plugin 2.8+ generates an additional aggregate report

Only generate an aggregate report if there is more than one project in the reactor.
There are more things that can be done, like checking if there are any files in the modules. There's already a TODO for that in the code.

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1438951 13f79535-47bb-0310-9956-ffa450edef68