script: Add env CONTROLLER_DOMAIN for boot_docker

Signed-off-by: Wang Baoling <chinamerp@gmail.com>
This line is quite long, it should be split over multiple lines (see [here](https://google-styleguide.googlecode.com/svn/trunk/shell.xml#Line_Length_and_Long_Strings)).

Also, we should avoid duplication and set `CONTROLLER_DOMAIN` & `DEFAULT_ROUTE_DOMAIN` in `main()` and reference here and in `boot_libvirt_lxc()`:

```bash
docker run \
  --env DISCOVERD=$ip:1111 \
  --env CONTROLLER_DOMAIN="${CONTROLLER_DOMAIN}" \
  --env DEFAULT_ROUTE_DOMAIN="${DEFAULT_ROUTE_DOMAIN}" \
  flynn/bootstrap
```