[MPOM-59] enable checkstyle:check on every build

git-svn-id: https://svn.apache.org/repos/asf/maven/pom/trunk@1628078 13f79535-47bb-0310-9956-ffa450edef68
use property to exclude files, since it can more easily be overridden in child poms

git-svn-id: https://svn.apache.org/repos/asf/maven/pom/trunk@1628081 13f79535-47bb-0310-9956-ffa450edef68