XMPP: Replace host list instead of adding during domainname change

Created by: SunilMohanAdapa

In ejabberd configuration, whenever we change the domainname, the new domain is added to the host list in ejabberd.yaml. The old hostname even though invalid was kept because of the way deal with users. Currently, we use LDAP for user management and we probably don't need to keep the old domainname in the host list anymore. Explore and implement removing the old domainname and replace it with new domainain.