gentoo-iproute2: Add sys-apps/iproute2 to images

Avoid:

  lxc-start: failed to open log file "/var/lib/lxc/b149.../b149....log" :
    No such file or directory

because iproute2 needs to be in the image [1].

Really?
  https://github.com/dotcloud/docker/issues/925
  https://github.com/dotcloud/docker/pull/2157

[1]: https://groups.google.com/forum/#!msg/docker-user/yXOt3qISHXU/8ntkUNJR0h0J