generate certs if REGISTRY_TLS_VERIFY

Usage:
```
docker run -e REGISTRY_TLS_VERIFY=1 -e REGISTRY_COMMON_NAME=$(boot2docker ip):5000 \
  -v /etc/docker/certs.d:/certs.d -p 5000:5000 \
  -e GCS_BUCKET= -e GCP_OAUTH2_REFRESH_TOKEN= \
  google/docker-registry 
```

while waiting for https://github.com/docker/docker-registry/pull/693 to be merged.

/cc @tiborvass @dmp42 @dlorenc @ktintc @jbeda 
only generate if GUNICORN_OPTS is empty.