Make it possible to run the changes-check and changes-validate goals only once for a multi-module project

If the pom.xml contains modules, the goals changes:changes-check and changes:changes-validate is run for each module, generating warnings because it can not find the changes.xml file.

[INFO] --- maven-changes-plugin:2.8:changes-check (default-cli) @ module ---
[WARNING] The file H:\project\module\src\changes\changes.xml does not exist.

It should be possible to configure the POM to run these goals only for the root project. It works if you use "mvn -N" option, but we need to be able to configure this behaviour in the pom.xml.