Since the switch from `-v` to `--mount` (in 1fca917ad1),
we've regressed when `matrix_ssl_retrieval_method == 'none'`.

In such a case, we don't create `/matrix/ssl` directories at all
and shouldn't be trying to mount them into the `matrix-nginx-proxy`
container.

Previously, with `-v`, Docker would auto-create them, effectively hiding
our mistake. Now that `--mount` doesn't do such auto-creation magic,
the `matrix-nginx-proxy` container was failing to start.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/734