Refactor fabfile

- This partly addresses #4, by using the public docker registry, instead of a private one.
- Pull the hostname to use from the config file (DRY).
- Use `DEBIAN_FRONTEND=noninteractive`. (There is a grub update that wants input, now)
- #10
In fact, because it is a non-supported link syntax, there is an angle bracket at the end in the rendered version, making the link incorrect