add etherpad to nginx wanted services

it's required if enabled by the dimension config here:
6eb8fb8392/roles/matrix-etherpad/tasks/init.yml (L42-L49)

see also #1517

group_vars/matrix_servers

@@ -1482,6 +1482,8 @@ matrix_nginx_proxy_systemd_wanted_services_list: |
     (['matrix-jitsi.service'] if matrix_jitsi_enabled else [])
     +
     (['matrix-bot-go-neb.service'] if matrix_bot_go_neb_enabled else [])
+    +
+    (['matrix-etherpad.service'] if matrix_etherpad_enabled else [])
   }}
 
 matrix_ssl_domains_to_obtain_certificates_for: |