SUREFIRE-999: Skip phase `validate` during site-generation for `report-only` resp. `failsafe-report-only`

* We have a lot of projects in our CI system (Jenkins) where we run `mvn clean deploy site-deploy`.
* Because of this, we do not repeat test execution again during phase `site` by defining the `reportSet` to include `report-only`.
* However, `report-only` does invoke the phase validate again which will execute some lengthy enforcer checks.