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>