Serve a TLS endpoint if REGISTRY_TLS_VERIFY is set and GUNICORN_OPTS is not

If `REGISTRY_TLS_VERIFY` is set, but `GUNICORN_OPTS` is not, then serve via a TLS endpoint instead of plain HTTP.

This is done by setting `GUNICORN_OPTS` to some default value, expecting the following files to be present:

* /ssl/ca.crt
* /ssl/registry.cert
* /ssl/registry.key

Signed-off-by: Tibor Vass <teabee89@gmail.com>
the same discussion was posted above: https://github.com/docker/docker-registry/pull/693#issuecomment-62487926