There was a `matrix_nginx_proxy_enabled|default(False)` check, but:
- it didn't seem to work reliably for some reason (hmm)
- referring to a `matrix_nginx_proxy_*` variable from within the
  `matrix-synapse` role is not ideal
- exposing always happened on `127.0.0.1`, which may not be good enough
  for some rarer setups (where the own webserver is external to the host)