Add support for enabling/disabling the pubsub service

Description

Use a system property to enable or disable the service. We do not have a UI page for pubsub so this setting will only be available for now as a system property. There will not be a need to restart the server after changing this setting. The new system property will be: xmpp.pubsub.enabled

Environment

None
Fixed

Assignee

Gaston Dombiak

Reporter

Gaston Dombiak