Read list of admin users from backend

When using Clearspace we should skip the admin setup page and just ask Clearspace for its list of admins. Moreover, every time in CS the list of admins is modified we should update it in Openfire (if we are caching them). When not integrated with Clearspace we should store the list of admins in the database (instead of conf/openfire.xml) so that all cluster nodes share the same list of admins.