[WFLY-122] : Clean unreferenced items from the content repository

Contents in content repositories are now cleaned. We are looking for all referenced contents in the model :
 * deployments
 * overlays
 * rollout-plans
All contents that are not referenced will be removed.

Jira : https://issues.jboss.org/browse/WFLY-122
We chatted in hipchat, but I'll summarize here in case anyone reads this in the future.

Managed domain servers do not need to maintain their own content repo; they use the HC's repo.

Since they don't own their own repo, no logic to clean the repo should execute on managed domain servers.