Add new env variable HTTP_PORT to setup port forward in docker network.

Docker container network does not support random port mapping. 
Add a new env variable HTTP_PORT after merge #371.
```
$ git clone https://github.com/OpenBazaar/OpenBazaar && cd OpenBazaar
$ sudo docker build -t=test/ob .
$ sudo docker run -d -e "HTTP_PORT=8899" -p 8080:8899 -p 12345:12345 test/ob
$ sudo docker ps
CONTAINER ID        IMAGE               COMMAND                CREATED             STATUS              PORTS                                              NAMES
8bad25d8bfd1        test/ob:latest      "/bin/sh -c 'IP=$(/s   3 seconds ago       Up 2 seconds        0.0.0.0:8080->8899/tcp, 0.0.0.0:12345->12345/tcp   boring_almeida
``` 

Is there any way to disable node/tornadoloop.py open_default_webbrowser() for headless computer ?