merge integration stack content into discovery site CI builds

Today, we only merge IS content into the discovery site builds during staging and release.

But since this merge operation is now 100% scripted, we could trigger it for all CI builds too.

See https://dev-platform-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/view/Devstudio/view/jbosstools-releng/job/jbosstools-releng-push-to-staging-04-update-merge-composites-html/ for sample of how to run the merge.