Provide a way to add enable rpc authentication for transmissionvpn.

[cjcharles777]

There should be a way to enable the rpc authentication in transmissionvpn. Just like in http://haugene.github.io/docker-transmission-openvpn/faq/#how_do_i_enable_authentication_in_the_web_ui. Currently at the moment the transmission vpn container is created but can't be behind a login of any sort.