some notes to the changes:
* mod_event_pusher_push:maybe_enable_node/5:
   - first of all, previous implementation had problems with federation pub-sub servers.
   - also, having dynamic domains enabled, it might be too expensive to get all the domains.
   - and that dynamic domains are added in the lazy manner to the routing table
* rework of the virtual_pubsub_hosts option processing/validation:
   - @hosts@ template was a bit confusing, people were asking about the difference to @host@ template all the time.
   - it is still possible to configure manually all the sub-hosts, if there's such a need.
   - now processing of the virtual_pubsub_hosts is done in the same way as other host templates
   - there is no need in storing separate normalized_virtual_pubsub_hosts option, so we can remove gen_mod:set_module_opt/4 call