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

[mfriedenhagen]

• 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.

[mfriedenhagen]

@krosenvold, I wanted to rebase upstream and give a better explanation and this did destroy your comments, sorry about this :-(.

[mfriedenhagen]

Use annotation based solution.