Porxy does not allow Porta's master domain to be configured

[guicassolato]

3scale Porta's master domain should be configurable in Porxy, rather than hard-coded at

https://github.com/guicassolato/porta-dev-tools/blob/df2a9672c4792d895f498af8846155281a411f9a/porxy/nginx.conf#L8-L9

and

https://github.com/guicassolato/porta-dev-tools/blob/df2a9672c4792d895f498af8846155281a411f9a/porxy/docker-entrypoint.sh#L8